# Superconductivity for railway applications: A review

Michael Rudolf Koblischka, Kévin Berger, Hervé Caron

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Michael Rudolf Koblischka, Kévin Berger, Hervé Caron. Superconductivity for railway applications: A review. Energy Conversion and Management: X, 2025, 28, pp.40, 101344. 10.1016/j.ecmx.2025.101344. hal-05314155

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# Superconductivity for railway applications: A review

Michael Rudolf Koblischka<sup>a ID,*</sup>, Kévin Berger<sup>a</sup>, Hervé Caron<sup>b</sup>

## ARTICLE INFO

**Keywords:**

Railway
Superconductivity
Levitation
Maglev
Wheel-rail contact
Transformers
Traction motors
Feeder cables
Electric grid
SDGs

## ABSTRACT

This review explores the application of superconductivity in the railway industry, encompassing both magnetic levitation (maglev) systems and innovations in conventional electrified railways. As the world faces urgent challenges related to climate change, energy efficiency, and the transition to renewable energy, superconductivity offers a promising path forward by enabling loss-less current transport, reducing emissions, and optimizing energy use in rail systems. Superconductivity aligns closely with several United Nations Sustainable Development Goals (SDGs), specifically for SDG 07: Affordable and clean energy, SDG 09: Industry, innovation, and infrastructure, SDG 11: Sustainable cities and communities and SDG 13: Climate action. The review incorporates a bibliometric analysis using the Clarivate Web of Science, identifying 116 scientific articles focused on the intersection of superconductivity and railway technology. The analysis highlights the leading research contributions and top-cited authors, the key institutions driving innovation and the predominant research themes and micro-citation trends. These findings provide valuable insight into the technological landscape, enabling stakeholders to track emerging topics, identify collaboration opportunities, and support strategic planning towards sustainable railway development. The paper also discusses the progress of superconducting materials tailored for large-scale transport applications. High-temperature superconductivity (HTS), operating at liquid nitrogen temperatures, is particularly emphasized due to its potential for loss-free current transport, higher energy efficiency and compact, high-performance components for railway use. These benefits are strongly linked to the global attention focused on climate change, carbon reduction, and the transition to sustainable energy systems. Superconductivity in railway systems is explored in two main research directions, covering both superconducting maglev systems and conventional electrified railways. For maglev systems, the three fundamental levitation principles – Electromagnetic Suspension (EMS), Electodynamic Suspension (EDS), and HTS-based flux pinning – are outlined. Major international developments and future maglev innovations are discussed, highlighting the next generation of high-speed rail driven by superconductivity-based maglev systems. In the case of conventional, electrified railways, superconducting technologies are increasingly applied to both onboard systems, including transformers, motors, eddy-current brakes and contactless power supplies, and the supporting infrastructure, comprising superconducting feeder cables, energy storage units, and grid stabilization systems. Practical implementations, such as superconducting DC feeder cables, already demonstrate both technical feasibility and commercial potential for enhancing efficiency and supporting large-scale railway electrification.

## 1. Introduction

The present-day world faces significant challenges and opportunities in the realm of energy, particularly with regard to climate change awareness and the transition to renewable energy sources. These developments include commitments to reducing carbon footprints and increasing energy efficiency. To address these global challenges, the United Nations established the Sustainable Development Goals (SDGs) in 2015, which were ratified as an international agreement aimed at improving global sustainability by the year 2030 [1,2]. These SDGs

also represent a challenge for further materials and engineering developments as discussed in Refs. [3,4].

Rail transport on electrified railways is one of the most energy-efficient and environmentally friendly modes of transportation, enabling a much higher traffic density compared to diesel-powered systems. This holds true for both freight and passenger services. The daily commuting of passengers in major cities worldwide would not be feasible without electrified railways, including extensive subway and metro networks. Electric railway traction has a long and significant

<footnote><sup>a</sup> Université de Lorraine, GREEN, F-54000, Nancy, France</footnote>

<footnote><sup>b</sup> SNCF Réseau, Direction Générale Industrielle et Ingénierie, F-93418, La Plaine Saint Denis, France</footnote>

<footnote>* Corresponding author.
E-mail address: michael.koblischka@univ-lorraine.fr (M.R. Koblischka).</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Possible uses of superconductivity in the field of railways/ground transportation. Superconducting levitation (maglev) trains will be discussed in Section 4 and the applications of superconductivity for conventional, electrified railways in Section 5. The violet arrow indicates that the progress in EDS and (flux) pinning maglev operations may influence especially the onboard use of superconductivity in conventional railway vehicles. This issue will be discussed in detail in Section 6 below.</figcaption></figure>

history, dating back to 1879 when Werner von Siemens demonstrated the first electric locomotive at an exhibition in Berlin. This was soon followed by the launch of the first electric tramway in 1881 in Berlin Groß-Lichterfelde [5]. If current trends continue [6], both passenger and freight rail activities are projected to more than double by 2050, with electric energy playing a crucial role in meeting this growing demand.

How does the research in superconductivity enter this field? Nishi-
jima et al. [7] already reviewed the role of superconductivity in ad-
dressing global environmental challenges and highlighted several ap-
plication areas where superconductivity could have a significant pos-
itive impact on the environment. These include water purification,
power distribution and storage, low-impact transportation, environ-
mental sensing, and other related fields. In this context, the use of
superconductivity – with its exceptional capability for lossless energy
transmission – is particularly well-suited to contribute to various appli-
cations in railway systems in respect to SDGs 07, 09, 11 and 13, see
the recent review by Duran et al. [8].

The field of superconductivity has seen tremendous efforts of re-
searchers, especially since the discovery of the high-T<sub>c</sub> superconduc-
tivity (HTS) [9] which enables operation at elevated temperatures,
e.g., at the boiling temperature of liquid nitrogen (abbreviated LN<sub>2</sub>,
T = 77 K). Firstly, silver-cladded (Pb,Bi)<sub>2</sub>Sr<sub>2</sub>Ca<sub>2</sub>Cu<sub>2</sub>O<sub>7</sub> (in the following,
abbreviated as Bi-2223) tapes with Pb doping were developed, called
the first generation HTS wires [10], which already enabled the setup
of various demonstrators including hundred meters of superconducting
cables operating in power stations [11,12]. The second generation of
superconducting tapes is based on REBa<sub>2</sub>Cu<sub>3</sub>O<sub>7</sub> (in the following, ab-
breviated as (RE)BCO, with RE denoting rare earth elements, Y, Nd, Gd,
Eu), which are fabricated using ablation processes like MOCVD or laser
ablation on Ni tapes with the help of some buffer layers to optimize
the growth conditions of the (RE)BCO layer. These tapes, often also
called coated conductors [13,14], exhibit much higher critical current
densities than the Bi-2223 tapes, especially at elevated temperatures
like 77 K, and have enabled many new applications in electric motors,
busbars, and cables [15]. With such material available in long lengths,
practical applications also in the railway sector are now feasible. More
details of the superconducting materials are discussed in Section 3 of
this review.

First of all, it is important to highlight here that the potential
applications of superconductivity in railway systems are twofold (see
the schematic sketch of Fig. 1):

(i) Magnetic levitation (maglev) – this approach employs conven-
tional or superconducting magnets to levitate the train above
the tracks, thereby eliminating wheel-rail contact and friction.
The area of maglev splits up in two directions, the supercon-
ducting electrodynamic suspension (EDS) maglev and the HTS
(flux)-pinning maglev. Maglev enables exceptionally smooth and
high-speed travel, but also effective mass transport (e.g., peo-
ple movers) and on smaller scale, precise delivery of objects in
industrial environment [16–18].

(ii) Conventional electrified railways – here, superconductivity can offer innovative solutions for electric motors and transformers within the trains themselves. Additionally, the application of superconductivity has the potential to transform the supporting railway infrastructure through the use of superconducting feeder cables [19], superconducting magnetic energy storage (SMES) systems [20], fault current limiters (FCL) [21], and wireless power transfer technologies for supplying electric power to moving vehicles.

Notably, advances in EDS and HTS pinning maglev technology are poised to accelerate the adoption of onboard superconducting systems also for conventional electrified railway, thus translating high-performance innovations into broader railway applications as indicated by the violet arrow in Fig. 1.

Reaching high speeds is required to compete with other trans-
portation means like aircraft, thus several attempts were made to
set new record values for both maglev and conventional railways.
Striking results were achieved in this way, but the typical speeds of
conventional high-speed trains (TGV, ICE, Shinkansen) are remaining
currently between 250 and 300 km/h [22,23], whereas for the future
superconducting JR-maglev (Chuo-Shinkansen) in Japan, the aim is at
an operational speed of 500 km/h, enabling a 1 h travel time between
the terminals in Tokyo and Osaka [23].

However, the development of high-speed trains is only one aspect of
the various applications where superconductivity can play an important
role. Moving a larger amount of freight and people causes an increasing
<figure><img src="image_5.png" /><figcaption>Fig. 2. (a) Comparison between maglev and other different transportation means concerning the unit carbon dioxide-equivalent emissions and (b) presenting the unit energy consumption.</figcaption></figure>

Source: Reprinted with permission from Dong et al. [24].

energy consumption, which brings the existing power network to its limits. Fig. 2(a) presents an analysis of the equivalent CO₂-emission as a function of the cruising speed for various transportation means, and (b) the unit energy consumption, which attests to an outstanding position for the superconducting maglev [24].

High-temperature superconducting power cables offer significant potential for addressing the increasing energy needs of rail transport. For example, superconducting power cables are more compact than conventional copper cables, and thus, can enable efficient transfer of electrical power between adjacent traction substations even in limited space. This capability can reduce the need for additional substations and minimize infrastructure upgrade costs. Moreover, superconducting cables can deliver the high power density required to expand railway electrical grids in densely populated urban areas – a major advantage for grid operators considering the adoption of this advanced technology. Furthermore, the application of SMES [20] and FCL [21] technologies can contribute to a stable power network with continuous and safe delivery, even during rush hours and peak travel times.

The aim of this review is to provide a collection of present-day superconductivity applications in the railway sector, both for maglev and the conventional electrified railway systems. In both sectors, several projects are close to practical use, fulfilling the demands of the UN Sustainable Goals.

This review is organized as follows: Section 2 presents the bibliographic analysis of all publications identified in the Clarivate Web of Science (WoS) that address the intersection of railway systems and superconductivity. In Section 3, a summary of the superconducting materials for applications is given. Section 4 discusses the application of superconductivity in the development and operation of magnetic levitation (maglev) trains and points out several points which still require intense research. Section 5 explores the potential applications of superconductivity in conventional electrified railways, both for on-board use and for the supporting infrastructure, including an analysis of the underlying principles and the potential advantages offered by these technologies. Finally, Section 6 distills the main conclusions and explores emerging opportunities for superconducting technologies in rail transport, highlighting pathways towards broader deployment, enhanced performance, and reduced environmental impact.

## 2. Bibliometric analysis of the topic

We start our review with a bibliometric analysis of all articles published in the literature.

As a basis for the bibliometric analysis of this topic, the Clarivate Web of Science (WoS) database was employed [25]. The initial search was performed using the query “railway AND supercond*”, where the asterisk acts as a wildcard. This search returned a total of 124 articles covering all publication years from 1994 to 2025. The year

<figure><img src="image_6.png" /><figcaption>Fig. 3. The number of articles in the field railway and superconductivity as a function of the publication year.</figcaption></figure>

<caption>Table 1</caption>

Summary of the highly cited articles, review articles and the most cited articles for the WoS search “railway* AND supercond* NOT ”railway switch“ NOT freeway NOT magnetocardiography”.

<table><thead><tr><th>Authors</th><th>Ref.</th><th>Year</th><th>Type of article</th><th>SDGs</th><th>Citations</th></tr></thead><tbody><tr><td>Fu et al.</td><td>[27]</td><td>2024</td><td>highly cited</td><td>none</td><td>22</td></tr><tr><td>Chen et al.</td><td>[28]</td><td>2024</td><td>highly cited</td><td>none</td><td>24</td></tr><tr><td>Mousavi et al.</td><td>[29]</td><td>2017</td><td>highly cited</td><td>07</td><td>306</td></tr><tr><td>Dominguez et al.</td><td>[30]</td><td>2025</td><td>review</td><td>11</td><td>7</td></tr><tr><td>Mousavi et al.</td><td>[29]</td><td>2017</td><td>review</td><td>07</td><td>306</td></tr><tr><td>Takagi</td><td>[31]</td><td>2010</td><td>review</td><td>11</td><td>16</td></tr><tr><td>Mousavi et al.</td><td>[29]</td><td>2017</td><td>review</td><td>07</td><td>306</td></tr><tr><td>Ise et al.</td><td>[32]</td><td>2005</td><td>article</td><td>07</td><td>158</td></tr><tr><td>Stephan et al.</td><td>[33]</td><td>2012</td><td>article</td><td>none</td><td>58</td></tr><tr><td>Maeda et al.</td><td>[19]</td><td>2019</td><td>article</td><td>none</td><td>52</td></tr><tr><td>Oomen et al.</td><td>[34]</td><td>2003</td><td>article</td><td>none</td><td>45</td></tr><tr><td>Lee et al.</td><td>[35]</td><td>2014</td><td>article</td><td>none</td><td>45</td></tr></tbody></table>

1994 marks the appearance of the first article concerning railway and superconductivity, dealing with an active filter system for maglev operation [26].

However, this result included several articles that are not directly relevant to the intended focus. For example, multiple papers discuss the “railway switch model” [36], which was developed to explain current flow in textured Bi-2223 superconducting tapes and its extensions, but do not relate to railway applications. A further development of this model was called “freeway” [37], which also must be excluded.
Additionally, some entries primarily addressed topics such as “magnetocardiography” and the development of sensors for this purpose, which are unrelated to railways. To refine the results, the search term was modified to “railway* AND supercond* NOT “railway switch” NOT freeway NOT magnetocardiography”. Note the quotation marks around “railway switch” which are needed to search for the complete wording. This revised query returned 113 articles. One further article, purely focused on Materials Science and was authored by a member of RTRI, Tokyo, was excluded from the present search as it did not address any railway-related aspects. Therefore, the final dataset comprises 112 relevant articles covering superconductivity applications in railways. An additional four articles published in 2025 (as of June 1st, 2025) were added, resulting in a total of 116 articles to be reviewed here. Fig. 3 presents the number of articles found in the WoS search as a function of the publication year. After a slow start, the number of publications has reached a level of 10–12 papers/year since 2020, which is quite good for such a narrow research topic. Among these, 38 articles explicitly mention the term “maglev” in connection with the keyword “railway”, so 78 articles discuss the use of superconductivity for applications in conventional railway systems, which is the main interest of the present review. Of course, there is a much larger amount of articles concerning the development of maglev, but the vast majority of these articles discuss basic features like levitation forces, stability, magnetic drag, etc., so there is not always a direct link to the railway-like operation of maglev systems.

**Table 1** summarizes the details of three types of articles in the search: There are 3 highly cited papers (i.e., papers with many citations in a short time) listed in WoS [27–29], 3 review articles [29–31] and the most cited articles. The highly cited articles as tagged by WoS deal with flywheel technology [29], ultra low-loss cables for railways [28] and a proposal for a liquid hydrogen-electricity hybrid energy transmission system for railways [27], which takes advantage of the recent development plans for a hydrogen economy in various countries. This represents indeed a quite ‘hot’ topic. In contrast, the review articles listed here focus on the use of energy storage systems for railway applications in general [30], the flywheel energy storage technology in particular [29] and the possible energy saving techniques for the power feeding network of electric railways [31]. It is noteworthy that the five most-cited articles comprehensively cover the various applications of superconductivity in railway networks. Further, we must note here that only the two top-cited articles have received a SDGs-tag, whereas the others are tagged with 5.30.21 Critical Current Density. All other publications in this area have received 40 or fewer citations, highlighting the relatively narrow scope and specialized nature of this research field.

Another interesting insight comes from the 19 Open Access (OA) articles, which account for 16.4% of all published papers [25]. This proportion highlights the growing emphasis by research funding organizations on making research outputs freely accessible, reflecting an increasing demand for transparency and widespread dissemination in the field.

**Fig. 4(a)** presents the distribution of the articles to the countries of the authors. Here, Japan (57 articles) and P.R. China (34 articles) are leading by far, followed by South Korea (8 articles) and France (7 articles). All other countries listed here score only 1–4 articles in this search. Very interesting is then the analysis of the so-called micro citation topics provided by WoS as shown in Fig. 4 (b). 11 such micro citation topics were identified by our search. The dominating topic is 5.30.21 Critical Current Density with 54.8% of all articles, followed by 7.192.1197 Wheel-Rail Contact with 4.1%, which is due to maglev research, and 3.4% for 4.183.2127 Train Scheduling. 2.7% each go to 4.58.1042 Wireless Power Transfer, 4.18.101 Power Quality, and 4.18.136 Induction Motor, which all cover the conventional electrified railways. Here, we must note that 5.30.21 Critical Current Density was very recently (June 2025) renamed by WoS to 5.30.21 High-$T_c$ Superconductivity [25]. Applying the WoS SDGs filter to the results of

the search yields 13 articles for SDG 07 (*Affordable and Clean Energy*), 13 articles for SDG 11 (*Sustainable Cities And Communities*) and 2 for SDG 13 (*Climate Action*). So, in total, 28 articles have received an SDGs tag, which corresponds to 25% of all articles in this subfield. This is a better relation as obtained for the search “supercond*” [38], where only 18% of all published articles were found to receive an SDGs tag. On the other hand, the 25% are much less as compared to fields like Magnetism, Nano or Microscopy, which can reach up to 70% [39]. However, all the articles found in our search clearly cover important applications which are in line with the SDGs as mentioned before, so it is difficult to understand the low SDGs tagging rate.

An analysis of the micro citation topics after applying the WoS SDG filter should bring out more information to this open question.

Fig. 4(d) shows that only 7 micro citation topics are left, and from these, 4.183.2127 Train Scheduling (35.3%) and 4.58.1042 Wireless Power Transfer (23.5%) are dominating.

The topic 5.30.21 Critical Current Density is completely skipped here, which implies that for WoS the research considering high-$T_c$ superconductivity is not counting in direction of the SDGs, which is a very astonishing result regarding the importance of the possible applications for daily life in the future. This was already recognized in Ref. [38], where it was concluded that this not-counting of HTS is indeed a big challenge for the field of superconductivity.

**Fig. 4** (e) and (f) present the top-authors with the most publications (e) and the top-institutions for our search. Here, Japanese researchers are clearly leading the worldwide efforts, with Masaru Tomita working at RTRI, Tokyo, being the top-author and his institution the top-institute. Fig. 4(e) reveals that the first 8 positions go to Japanese researchers, followed by Chinese researchers and only 3 European researchers from France are mentioned in the graph. A similar picture is obtained for the institutions (Fig. 4(f)), where besides the Japanese and Chinese institutes, only the University of Lorraine (5), SNCF Reseau (4) and Siemens AG (3) from Europe can be found. Remarkable is further that the contributions of Siemens stem all from 2003, summarizing projects of the period 2000–2003, and since then, no further contributions to the literature were made to this topic by this company.

Analysing the 116 articles for the publisher names reveals that the most important journal in this field is IEEE Transactions on Applied Superconductivity (IEEE TAS), which publishes the proceedings of the Applied Superconductivity Conference (ASC), taking place in two-year intervals. In recent years, the journal was also often chosen to publish the proceedings of the EUCAS, the European counterpart of the ASC. Thus, 60 articles of the total 116 articles (= 51.7%) were published in IEEE Trans. Appl. Supercond., leaving 56 articles for all other journals.

This shows that the field of railway and superconductivity is a relatively narrow one, and both the EUCAS and ASC conferences enable a gathering of the entire community. As already mentioned, the amount of literature on the topic “levitation” is much bigger, but considers a much broader spectrum of applications as well as basic research. This is also reflected in the reference list of the present review, comprising 240 entries as compared to the 116 articles found in the search “railway AND supercond*”.

### 3. Superconducting materials for applications

The first maglev systems studied made use of multifilament wires of NbTi and Nb$_3$Sn, and thus, required the cooling by liquid Helium (LHe, $T = 4.2$ K) [10]. The NbTi ($T_c = 9.6$ K, upper critical field $H_{c2} \sim 11$ T, 2 K) and Nb$_3$Sn ($T_c = 18$ K, $H_{c2} \sim 25$ T) wires still see continuous development, mostly driven by the needs for large magnet systems in accelerator and fusion research units [45–49]. Thus, the existing technology is well suited also for applications in railway systems, but on the cost and complications of the required LHe cooling. This is the main reason why the development of prototype systems for railway applications was delayed until the advent of materials enabling easier
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Bibliographic analysis of the search "railway AND supercond*". (a) The number of scientific publications shown for the various contributing countries. (b) The WoS micro citation topics. Note here the dominating topic "Critical Current Density". (c) The number of articles which obtained SDGs tagging. 3 SDGs (07, 11 and 13) are covered. (d) The micro citation topics after applying the SDGs filter. (e) Presents the top-authors of the field with their number of articles. (f) Shows the top-institutes with the number of articles.</figcaption></figure>

and cheaper cooling systems, both for the use within vehicles and for
peripheral systems.

The invention of the HTS materials led to enormous efforts to de-
velop material designs suitable for applications. Silver-sheathed (Pb,Bi)-
2223 tapes were developed at the beginning ("first generation tapes"),
employing principles similar to the conventional, metallic supercon-
ductors [48,49]. The preparation steps for such tapes are schematically
given in Fig. 5 (a1-a3). On this base, long-length tapes of up to 1 km
length and with up to 120 filaments could be fabricated commercially

(see Fig. 5 (a2)), allowing various prototype machinery to be con-
structed [50]. Fig. 5 (a3) presents some cross sections of such tapes
fabricated by various companies (NST A/S with 19 superconducting fil-
aments [40], Sumitomo with 119 filaments [41], and Bruker BEST with
107 filaments) [42]. Despite their high T<sub>c</sub> of 105 K, the (Pb,Bi-2223)-
tapes are not suitable for use at 77 K for high-current applications
because of their low irreversibility line [51]. However, they work well
at 20 K and below, and the high upper critical field of more than 120 T
allows the fabrication of insert coils to Nb<sub>3</sub>Sn magnets to reach even
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** The three types of HTS materials intended for large-scale applications. (a1–a3): (Pb,Bi)-2223 tapes, (b1–b3): (RE)BCO coated conductors, and (c1–c3): (RE)BCO bulk materials. (a1) gives a schematic sketch of the powder-in-tube process to fabricate the (Pb,Bi)-2223 tapes, (a2) presents a ready reel with a km-long tape, and (a3) shows cross sections of the multifilamentary (Pb,Bi)-2223 tapes. Upper: NST A/S (19 filaments) [40], middle: Sumitomo (119 filaments) [41], lower: Bruker BEST (107 filaments) [42]. (b1) gives a sketch of the layer arrangement in a typical (RE)BCO coated conductor, (b2) shows the (RE)BCO layer on a Ni-tape, and (b3) presents a piece of a ready, Cu-coated coated conductor from THEVA [43]. (c1) is a commercially available (RE)BCO sample from CAN Superconductors with a diameter of 2.5 cm [44], (c2) a similar bulk sample, but with drilled holes in the centre, and (c3) presents a typical trapped field measurement using a Hall probe scanner (T = 77 K).</figcaption></figure>

higher magnetic fields. Nevertheless, the production of the (Pb,Bi)-based tapes has come to an end, with Sumitomo stopping production in fall 2024.

The second generation of tapes (2G) makes use of the well-suited properties of (RE)BCO, which provides good properties at the temperature of boiling nitrogen (LN₂, 77 K) [13–15]. Such tapes are fabricated on the base of a metallic substrate material like Ni as shown in Fig. 5 (b1). This requires buffer layer(s) applied by sputtering to provide good growth conditions for the HTS material. Then, the (RE)BCO layer is coated onto this using MOCVD with a thickness of ~1.6 µm. Such tape is presented in Fig. 5 (b2). An overlayer of Ag is then coated on both sides of the tape, followed by electroplating a 20-µm-thick Cu layer on both sides of the tape to ensure good thermal contact. A ready piece of such tape is shown in Fig. 5 (b3), fabricated by THEVA [43]. The high critical current densities, $J_c$, achieved by these (RE)BCO tapes or “coated conductors” now enable many applications using the cheap LN₂ coolant or cryo-cooling.

The REBCO material enables also another way of preparing samples for applications. These so-called “bulk” materials [52] are presented in Fig. 5 (c1–c3). These bulks consist of pressed (RE)BCO powder, which is then heat-treated with a seed crystal placed on top. In this way, well-oriented, single-crystalline growth is introduced which enables the flow of relatively high critical currents by avoiding the otherwise existing weak links between individual grains. Such bulk materials are then used to trap magnetic fields and can then, once activated, act as superconducting permanent magnets as long as they are kept cold. Figure (c1) presents a photograph of such a bulk with a typical diameter of 2.5 cm [44]. In (c2), a bulk with drilled holes is shown which represents an attempt to improve bulk oxygenation and to improve flux pinning. Finally, Fig. 5 (c3) presents a typical measurement of the trapped flux in such a bulk. For details and records of such trapped field measurements, see Ref. [53].

Of course, it is necessary to mention here that there are more materials suitable for application like MgB₂ and the iron-based superconductors (IBS). MgB₂ is the metallic superconductor with the highest

$T_c$ of 38.5 K, but was discovered only in 2001 [54,55]. Nevertheless, the fabrication of wires and tapes was fastly developed, and so, today this material can be produced commercially in long lengths [55]. Furthermore, MgB₂ is also well suited for the fabrication of bulk materials, thereby avoiding several problems of the (RE)BCO materials like oxygenation problems or cracking, and, very importantly, the use of expensive rare earth materials. Furthermore, the weight of the MgB₂ bulks is considerably reduced as compared to the (RE)BCO bulks. The foreseen operation temperature of MgB₂ is 20 K, which can be easily maintained with modern cryo-cooling systems. Thus, we may see soon also the use of MgB₂ materials in railway applications.

The IBS materials were discovered in 2008 [56], and soon, several families of these materials were known. Among them, the 122-family (e.g., (Sr,Ba)₀.₆K₀.₄Fe₂As₂) enabled the fabrication of 100-m-long, multifilamentary IBS tapes, mainly intended for low-temperature, high-field operation as the $T_c$ is about 38 K, but $H_{c2}$ may reach 100 T at 4.2 K [57]. Thus, there is still a lot of development required concerning the tape length and the further improvement of $J_c$ by introducing additional flux pinning centres. Therefore, the IBS tapes are an upcoming new material, but do not yet play a role for railway applications.

# 4. MagLev trains

## 4.1. The basic maglev principles

Magnetic levitation (maglev) trains enable high-speed, low-friction travel by using magnetic forces to lift and propel the train above the track. In a typical Maglev system, magnetic coils embedded along the track generate a strong magnetic field that interacts with magnets on the train, causing it to levitate without any physical contact. This eliminates wheel-rail friction, allowing for exceptionally smooth and quiet operation [16].

The currently commercially operating maglev systems do not use superconducting materials, but rely only on conventional magnet technology [58,59].
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** The 3 different principles of maglev operation, (a1,a2) electromagnetic suspension (EMS), (b1–b3) electrodynamic suspension, and (c1–c5) HTS (flux-)pinning maglev. (a1) The principle of EMS using conventional magnets, which is used in the Transrapid [58] and Shanghai airport [59] maglev trains. The right image shows the cross section; the magnets for levitation and guidance are indicated in red. The left image presents a longitudinal view (the grey dashed line indicates the position of it). The yellow arrow indicates the direction of movement of the magnetic field, the grey arrows illustrate that the car is pulled by the field. (a2) Transrapid 06, now exhibited at the German Museum, Bonn (Photo MRK). (b1) The principle of EDS maglev. The EDS is mainly employed by the Japanese maglev trains [60]. Suspension is achieved using superconductors on board of the vehicle. (b2) Propulsion is obtained via a linear motor system. (b3) Maglev vehicles exhibited at RTRI (Photo KB). (c1) HTS pinning maglev principle. The field lines produced by the PM guideway are arrested at flux pinning sites (red-yellow spots) within the superconducting bulk, leading to a passive, stable levitation as indicated by the orange arrows. (c2) Demonstration of stable levitation of a YBCO coated conductor *below* a PM [61]. (c3) Demonstration of stable levitation of a gold-coated, YBCO thin film *below* a PM [61]. (c4) Leviation of a model train *below* a PM guideway. (c5) The world first HTS pinning levitation train designed for high speeds.</figcaption></figure>

*Source:* Reprinted with permission from Ref. [62].

There are several reviews in the literature discussing the basics of superconducting maglev systems, operation and the current status of research and engineering [60,64–67]. A key advantage of maglev technology is its ability to reach speeds significantly higher than conventional trains – often exceeding 500 km/h – while maintaining excellent stability, reduced noise, and minimal vibrations [68–70]. Additionally, maglev systems offer a substantially lower carbon footprint compared to traditional diesel or electric locomotives, positioning them as a promising solution for sustainable high-speed transportation.

Firstly, we have to separate various maglev systems, including electro-magnetic suspension (EMS), electro-dynamic suspension (EDS),

and high-temperature superconducting (HTS) pinning maglev as shown schematically in Fig. 6 (a–c).

EMS systems (Fig. 6 (a1,a2)) use electromagnets mounted on the train, which are oriented upward towards a ferromagnetic rail made usually from steel. The electromagnets attract the train upward towards the rail, causing the train to levitate slightly below the track. The magnetic attraction varies inversely with the square of distance, *d*, so minor changes in *d* can cause large changes in magnetic force. As a result, these changes in force are dynamically unstable. Thus, a slight divergence from the optimum position tends to grow, thus requiring sophisticated, active feedback systems to maintain a constant target gap, i.e., the distance from the track, which is typically only about
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** A schematical sketch of the Hyperloop capsule magnetically levitated in an evacuated steel tube and the various acting forces on it. Removing the air from the tube enables high-speed running like aircrafts, with possible speeds around ~1000 km/h.</figcaption></figure>

*Source:* Redrawn from Ref. [63].

~15 mm [58]. Electromagnetic suspension offers frictionless travel but comes with significant engineering challenges due to the instability of magnetic attraction over small gaps. The solution lies in precise and responsive control systems that can adjust electromagnetic force in real time. On this base, the maglev trains of Transrapid on the Emsland test line in Germany [58] and the Shanghai airport maglev [59] were constructed, being the first commercially operating maglev system.

Despite the huge expectable benefits of this technology, certain technical challenges persist, particularly related to the stability of maglev trains when encountering track irregularities. Thus, many articles in the literature deal with stability issues (see e.g., Ref. [71]). For example, research by Zhai et al. [68] has shown that such irregularities can affect the suspension gap and potentially degrade ride quality at high speeds. To address this, solutions such as integrating accelerometers into the electromagnets have been proposed, aiming to improve real-time control and enhance performance and stability during operation.

In electrodynamic suspension (EDS) systems (Fig. 6 (b1–b3), magnetic fields are generated by superconducting magnets (as in the case of JR maglev) or by permanent magnets (e.g., Inductrack system) [72]. The train and track both generate magnetic fields. As the train moves, these changing magnetic fields induce currents in coils or conductive strips embedded in the guideway. These induced currents create a repulsive force (Lenz's Law [73]) that lifts the train. In some configurations, the train can be levitated only by repulsive force. In the EDS system, the superconductors are on-board of the vehicle and, in the current JR maglev trains made up from low-$T_c$ superconducting coils. An EDS system is naturally stable as any vertical displacement of the train leads to restoring forces: Moving closer causes stronger repulsion which pushes the train away. Moving farther leads to weaker repulsion forces, so gravity pulls the train back. Unlike EMS, no active feedback control is needed for levitation stability. However, at low speeds, the relative motion between train and track is insufficient to induce large enough currents to levitate the train.

Hence, a minimum take-off speed (typically 100 km/h or more) must be reached before levitation occurs. For operation at low speeds, wheels or landing gear are required. The JR-type maglev uses aircraft-style retractable wheels. The original attempts to use traditional railway bogies, which would have enabled a combination of maglev and Shinkansen stations, failed due to the mass and the mechanical risks they introduced at high speeds.

Another disadvantage of the EDS system is that it naturally generates a magnetic field in the track ahead of and behind the lift magnets. This field opposes the magnets and causes magnetic drag. Usually, this is only a concern at low speeds, which is one reason why JR decided against using a purely repulsive system and instead chose the sidewall levitation system as depicted in Fig. 6 (b2). At higher speeds, other

types of drag become more significant. For both EMS and EDS systems, propulsion is typically achieved using a linear motor setup.

HTS pinning maglev (Fig. 6 (c1–c5)) – see also the general discussion on levitation in Refs. [74,75] – refers to a magnetic levitation system that exploits flux pinning to achieve stable levitation without the need for active control. Fig. 6 (c1) provides a schematic sketch. In these systems, the guideway typically consists of permanent magnets – often arranged in a Halbach array to concentrate the magnetic field – while the HTS elements are mounted on the vehicle itself. When cooled below their critical temperature and exposed to the guideway’s magnetic field (that is, “field cooling” or FC), the superconductors trap flux lines via strong pinning centres (indicated by the red/yellow dots in the sketch), thereby “locking” the vehicle into a stable equilibrium relative to the track as indicated by the four orange arrows. This form of levitation is passively stable, enabling operation without sensors, active feedback, or control circuits. A wide levitation gap can be achieved, allowing tolerance for uneven tracks or larger air clearances. Remarkably, HTS pinning maglev works in any orientation – including vertical or inverted configurations – a feature often demonstrated in public outreach [61,76,77]. Fig. 6 (c2–c4) present some typical laboratory experiments: In (c2), a piece of a YBCO coated conductor levitates below a tiny Nd-Fe-B magnet, and (c3) demonstrates that even a commercial, gold-covered YBCO thin film can levitate below a magnet.

The guideway or track has a configuration of permanent magnets, often arranged in a Halbach array to enhance magnetic field strength on one side. The superconductors are placed in the vehicles. When the cooled superconductor passes over the magnets, flux pinning occurs, so a stable levitation is provided as the superconductor essentially “locks” itself in position relative to the magnetic field [78,79]. Key advantages include inherent non-control stability, ultra-low noise, energy efficiency, and environmental compatibility. These properties make HTS pinning maglev attractive for specialized transportation (e.g., people movers, urban maglev concepts), but also for contactless manipulation in industrial or laboratory settings, and educational demonstrations. Although commercial deployment remains limited to date, its passive control simplicity and futuristic aesthetic continues to generate significant scientific and technological interest, which now also lead to the development of a real-size HTS pinning maglev train system as depicted in Fig. 6 (c5) [62,80], see also Section 4.3 below.

The Hyperloop concept, first articulated by Musk in 2013, envisions an ultra-high-speed ground transportation system in which pressurized capsules travel within low-pressure tubes, thereby minimizing aerodynamic drag and enabling theoretical cruising speeds exceeding 1000 km/h [63,81]. A principle sketch of the concept is presented in Fig. 7. While the system conceptually makes use of magnetic levitation (maglev) technology to eliminate wheel-ground contact, the precise levitation approach remains under active investigation, with several
competing strategies under consideration. Propulsion is typically realized through linear induction motors, whereas levitation schemes range from passive magnetic suspension (e.g., using Halbach arrays of permanent magnets in conjunction with conductive track materials) to actively controlled superconducting or electromagnetic systems. This openness to multiple levitation strategies reflects both the diversity of industrial actors involved and the unresolved trade-offs between system cost, efficiency, stability, and scalability.

Since its inception, Hyperloop research and development have been pursued by several industrial consortia and academic groups. Creating a low-pressure tunnel system, effectively simulating near-atmospheric conditions at high altitude, reduces aerodynamic drag to the point that feasible travel speeds approach those of commercial aircraft. However, Hyperloop remains at an early stage of development: only scaled prototypes and demonstrator models have been constructed, and no fully operational system yet exists. Consequently, many engineering challenges remain unresolved.

Notably, the potential role of superconductivity has received growing attention, as HTS-based approaches may offer advantages in terms of levitation stability and efficiency. In particular, recent studies have analysed the electromagnetic drag forces between HTS magnets and the guideway infrastructure [83–85]. Feasibility studies have been reported in India, China, Korea, and the Middle East, often with governmental participation. Moreover, the SpaceX Hyperloop Pod Competition (2015–2019) significantly stimulated innovation among university teams, advancing capsule aerodynamics, propulsion, and braking technologies. Despite this progress, substantial challenges persist. Chief technical barriers include maintaining a stable vacuum over kilometer-scale tube lengths, ensuring passenger safety under high acceleration and emergency conditions, and designing track-switching mechanisms compatible with ultra-high speeds. Economic and regulatory barriers are equally pressing: infrastructure costs are projected to exceed those of conventional high-speed rail, while safety certification and international standards remain under development. A recent review of the Hyperloop literature can be found in Ref. [86]. At present, commercial development has shifted emphasis from passenger transport towards freight logistics, with Europe emerging as a focal point for regulatory alignment and technology standardization. While widespread deployment remains uncertain, Hyperloop research continues to provide valuable insights into low-pressure transport systems, advanced levitation methods, and high-efficiency propulsion technologies.

To date, the most important high-speed maglev systems operating commercially still rely on conventional electromagnetic coil technology for both levitation and propulsion [58,59]. Nevertheless, superconducting materials hold the potential to further revolutionize this mode of transportation, particularly through the development of superconducting maglev trains employing superconducting coils. In such systems, the onboard magnetic system is replaced by superconducting magnets, enabling stronger and more efficient magnetic fields compared to conventional technologies. An exhaustive overview of medium- and low-speed maglev operations worldwide is provided in Ref. [87]. However, it must be noted here that the broad introduction of maglev trains suffers from the incompatibility to conventional trains, which poses a problem in Europe with an existing, well-developed train system. On the other hand, the possible advantages of maglev concerning energy efficiency are outstanding. Overall, maglev trains represent a prominent application of magnetism and superconductivity in modern transportation, offering a glimpse into a future of high-speed, energy-efficient, and environmentally sustainable travel.

## 4.2. EDS maglev development

EDS maglev systems have been developed, implemented, and researched in various countries, including Japan, Germany, France, China, Brazil, and Russia [64–67,87], demonstrating their potential as a viable next-generation transportation solution. Continued research

and technological development are essential for further enhancing the reliability, safety, and cost-effectiveness of maglev systems worldwide.

The first superconducting maglev trains based on the EDS principle were developed in Japan, with a test-line at Miyazaki prefecture starting from 1977. From 1996, a new test-line at Yamanashi prefecture (west of Tokyo, close to Mount Fuji) started its operation. This line was intended from the beginning to be a part of the new Chuo-Shinkansen line between Tokyo and Nagoya/Osaka [23,60,64]. All these first maglev cars were equipped with superconducting coils based either on NbTi or Nb₃Sn, requiring onboard liquid Helium (LHe)-cooling. Several of these maglev cars are now exhibited at the RTRI Institute in Kokobunji, Tokyo, as presented in Fig. 8 (a–e). The present-day JR levitation train (improved L0 series maglev, which started operation in 2020) relies still on superconducting coils made from NbTi and LHe cooling. The Series L0 improved version has fully adopted an inductive power transmission method, which was developed as the specification for operational vehicles. This has eliminated the use of gas turbines, which were installed in leading cars for the original Series L0 [60].

This improved superconducting levitation train is presented in Fig. 9 (a–c), where (a) shows the train on a completed section of track in the Kofu basin, (b) gives details of the train and the guideway, and (c) presents the design of the interior, where membrane materials are used to reduce sound reflections inside [60]. Due to ongoing disputes in Shizuoka prefecture, the opening of the first section of the Chuo-Maglev-Shinkansen between Tokyo-Shinagawa and Nagoya is delayed to 2034 [88]. However, this delay will also have some positive effect as on July 25th, the car M10 (M for ‘middle’) was unveiled. This car – besides a further improved interior design – is the first one using (RE)BCO coated conductors for the onboard coils [89]. Thus, also the JR-maglev will adopt the HTS technology in the near future.

To analyse the damping and the characteristics of the various levitation systems, different types of prototype vehicles and rail systems were built up, and this type of research is not limited to Japan only.

Fig. 10 (a–d) present the efforts of CRRC Changchun Railway Vehicles Co., Ltd. (Changchun, P.R. China) [82]. Recently, CRRC started with the development of a new 200-meter straight test line featuring a HTS electromagnetic suspension (EDS) maglev vehicle, as illustrated in Fig. 10 (a,b)). Unlike the Japanese EDS maglev systems, which were originally built using low-temperature superconducting (LTS) wires, the new setup employs eight HTS bulks mounted symmetrically on both sides of the vehicle. To systematically address practical challenges across a range of operating speeds, a test vehicle was initially designed to operate at a low velocity of approximately 14 m/s. This approach prioritizes operational safety during early testing phases. Once the low-speed operational issues have been resolved, it is planned to upgrade the test line for higher-speed trials, enabling investigation of additional dynamic phenomena. Achieving and maintaining the rated velocity is particularly challenging due to limitations in the traction system, the current superconducting magnets, and initial vertical displacement tolerances, among other factors. The vehicle is constrained in planar motion by four guiding wheels.

During a test run at a speed of 13.7 m/s, both the vertical displacement acceleration and the pitch angle exhibited beat vibration, as shown in the graphs given in Fig. 10 (c,d) [82]. This unexpected dynamic behaviour, called beat vibration, has not been reported in the literature before, especially at such relatively low speeds. These findings suggest that analysing only vertical displacement is insufficient. The phenomenon arises from the coupled effects of vehicle dynamics and complex magnetic interactions, and although it is specific to this vehicle configuration, similar behaviour could potentially manifest in other maglev systems. Thus, identifying and understanding this beat vibration is valuable for advancing the broader research and further development of EDS maglev technologies.

Finally, it is important to mention that EDS-based maglev trains hold the speed record of ground transportation with 603 km/h achieved
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Development of maglev trains in Japan: (a) The first superconducting levitation vehicle, ML-500, operating on a T-shaped magnetic rail. (b) The cryostat with the NbTi superconducting coil system of the MLU002, exhibited at RTRI. (c) Side view of the MLX01-3 car; the grey section at the front indicates the location of a cryostat system similar to (b), which has been removed from the car. (d) Interior of the MLX01-3, currently serving as an information centre at RTRI. (e) Front view of the MLX01-3. (f) The MLX01 maglev train operating at the Yamanashi test line (10 July 1998). (g) MLX01-901 maglev car with improved aerodynamics. Photograph (c) by KB, all others by MRK.</figcaption></figure>

on April 21, 2015 by a Series L0 improved version on already finished parts of the Chuo Shinkansen line [60]. Such advancements directly contribute to sustainable development goals related to infrastructure modernization, technological innovation, and climate action.

### 4.3. HTS pinning maglev development

With the discovery of the HTS, new possibilities came into play in the form of the HTS flux pinning maglev [79]. Whereas all first superconducting maglev trains used coils made from conventional superconductors, the development of bulk (RE)BCO HTS [52] led to the so-called trapped-field (TF) magnets [53], which may considerably reduce the weight of the superconducting system and thus, are very interesting especially for small-scale applications. The operation of TF-magnets will further enable skipping active and energy-consuming

charging equipment from the vehicles, as the TF-magnets must only be permanently cooled and activated only once. Such new approaches then required also considerable efforts in modelling the resulting magnetic fields of various arrangements of the TF magnets. The progress of developing HTS tapes from first generation to the second generation [93] enabled the present application of the (RE)BCO tapes for all purposes in a levitation train system. The formerly required LHe-cooling could be replaced by cryo-cooling systems, and the operation temperature may vary between ~20 K and 77 K, which again contributes to considerable energy savings.

Maglev trains using the flux-pinning suspension with HTS like (RE)BCO bulks, have demonstrated compelling advantages, especially in terms of acceleration and operational stability at high speeds [69, 94]. Compared to traditional high-speed trains based on intense magnets, HTS-based maglevs offer superior performance characteristics that
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** The Series L0 Maglev (improved version) of JR Central for the new Chuo-Shinkansen line. (a) The Series L0 Improved Version on the track in the Kofu basin, (b) gives details of the train and the guideway and (c) presents the design of the interior.</figcaption></figure>

*Source:* Photos obtained with permission from JR Central website [60].

align well with the requirements of modern high-speed railway systems. These systems reflect their potential as a viable future transportation solution.

The HTS pinning maglev research led in a first stage to the development of low-speed trains, similar to people movers. Typical examples of this are, e.g., the SupraTrans system [90,95] in Germany, and the Cobra maglev application in Brazil [33,96,97], see Fig. 11 (a-c), but also contactless transport and manipulation systems in laboratories and industrial settings (see e.g., Ref. [98]), and even advanced *vertical* transportation solutions like maglev-based (EMS) elevator systems for skyscrapers (e.g., [99]). Superconductors and magnets are thus paving the way for transformative progress in various transportation and mobility applications, underscoring their role as a key technology for sustainable and innovative future infrastructure.

SupraTrans (Fig. 11(a)) represents a pioneering transportation concept that leverages superconductive magnetic levitation to enable efficient, stable, and power-efficient passenger transit [90,95]. The goal of the project is to develop a fully operational prototype that integrates all critical subsystems required for real-world deployment, including propulsion, safety, positioning, power supply, transport logistics, and levitation. The levitation principle makes use of flux pinning in HTS bulks to achieve passive magnetic levitation. Unlike electromagnetic or electrodynamic maglev systems, SupraTrans requires no active electronic control systems to maintain levitation. The lateral and vertical positions of the train are inherently self-stabilized due to the strong flux pinning between the HTS materials and the magnetic track. The prototype system was recently moved from Dresden to Karlsruhe (KIT), and still receives further development [100].

The COBRA train (Fig. 11 (b,c)) is a superconducting maglev transportation system developed at the Federal University of Rio de Janeiro (UFRJ) [33,96,97]. Initiated in 2009, the project reached an operational milestone in 2018 with the establishment of a 200-meter test

track located within the University City campus. An operational line was set up and the test track successfully connected two buildings of UFRJ's Technological Center, transporting over 1000 students daily (see Fig. 9 (b,c)). The system relies on HTS-based magnetic levitation, offering a silent, energy-efficient, and self-stabilizing form of urban transport. The pilot unit demonstrated functional reliability and user capacity, leading UFRJ to pursue international certification. Despite its success, the COVID-19 pandemic halted further development and certification activities, temporarily pausing the project's momentum [89].

The HTS flux pinning maglev does, however, also enable a new version of high-speed systems. This development is further stimulated by the Hyperloop plans. While HTS maglev trains have demonstrated strong potential through engineering prototypes as shown before, several key challenges must be addressed before widespread high-speed operation becomes viable. This is especially true for the HTS (flux)-pinning maglev, representing the newest of the maglev systems.

After years of research, the HTS maglev team at Southwest Jiaotong University (Chengdu, China) launched the world's first high-speed HTS pinning maglev engineering prototype train [62,80], as shown in Fig. 6 (c5) and Fig. 12 (a,b). This vehicle represents the first HTS pinning maglev train explicitly designed for high-speed operation. The levitation principle is illustrated in Part I of Fig. 12(c): the levitation device, which replaces the wheelset, incorporates bulk YBCO superconducting pellets. Positioned at the field cooling height (FCH) above the permanent magnet guideway (PMG), the YBCO samples are cooled to their superconducting state using liquid nitrogen. As the train lowers under its own weight to the operating height, the changing magnetic field induces persistent currents in the YBCO bulks, which in turn interact with the external magnetic field of the PMG to generate levitation.

Achieving stable levitation at high speeds, however, introduces several technical challenges. Among these are high-frequency magnetic fluctuations arising from track discontinuities, such as joints
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** The 200 m straight EDS maglev test line using HTS bulks at the CRRC plant. (a) shows the vehicle and (b) the guideway. (c,d) Experimental running results when the speed $v = 13.7$ m s$^{-1}$. (c) gives the vertical displacement acceleration $\ddot{x}(t)$, and (d) the pitch angle $\gamma(t)$ as function of time, $t$.</figcaption></figure>

*Source:* Reprinted with permission from Ref. [82].

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** HTS pinning levitation systems, designed for low-speed transportation. (a) The SupraTrans vehicle on its guideway [90]. (b,c) Views of the COBRA (Brazil) maglev system [33].</figcaption></figure>

*Source:* Images taken from [89].
<figure><img src="image_15.png" /><figcaption>**Fig. 12.** (a,b) HTS pinning maglev engineering prototype train (Southwest Jiaotong University, China). Demonstrates passive, cryogenically cooled levitation using (RE)BCO bulks mounted onboard and interacting with a permanent magnet guideway (PMG). (c) Cross-sectional view of the HTS levitation unit (labelled 'Part I'), consisting of (RE)BCO bulk superconductors submerged in liquid nitrogen, enabling flux pinning-based levitation. (d) Cross-sectional view of the electromagnetic compensation (EMC) subsystem ('Part II'), composed primarily of coil groups designed to actively compensate for levitation force variations, particularly under high-speed conditions with fluctuating magnetic fields. (e) Image of the track showing the arrangement of the PM guideway, the coil stator, the positioning device, the track for the emergency wheels and the eddy current brake conductor plate.</figcaption></figure>

Source: (a–d): Reprinted with permission from Ref. [62], (e): Reprinted with permission from Ref. [80].

in the PMG, which generate rapid variations in the local magnetic field [101]. These fluctuations cause alternating-current (AC) losses in (RE)BCO bulk superconductors, leading to undesired heating and potential degradation of levitation performance [102–104]. As a result, the levitation force may become insufficient to guarantee stable operation under dynamic conditions. Current HTS bulk performance therefore remains a limiting factor in sustaining reliable levitation at high speeds.

To mitigate these issues, several solutions have been proposed. First, an electromagnetic compensation (EMC) device [ Fig. 12 (Part II, d)] has been introduced to dynamically stabilize the levitation force by compensating for losses or gaps in magnetic support [101]. Second, efforts are directed towards the optimization of HTS bulk pellets, focusing on both material properties and structural design, with the aim of enhancing levitation capacity and reducing susceptibility to AC losses [101]. These advancements are critical steps towards realizing high-speed, stable, and efficient HTS maglev systems. They remain at the core of current research and development in superconducting transportation technologies, as summarized in Fig. 12 (a–e).

A common feature among all maglev lines is that the ground in-
frastructure requires only the installation of permanent magnet tracks
and linear motors, resulting in a comparatively simple and reliable
structure. On the ground, the motor stator, positioning devices, and
an emergency wheel track are integrated to ensure safe operation in
the event of levitation failure. In addition, eddy current brake plates
are incorporated into the guideway to enable contactless braking and

enhance safety performance as discussed in Ref. [80]. The overall
composition of the track is illustrated in Fig. 12(e).

The unexpected results reported in Ref. [82] highlight the necessity
of monitoring the behaviour of a running maglev train. In particular,
there is a lack of research that accurately simulates the real load condi-
tions experienced by a high-temperature superconductor (HTS) maglev
train, especially with respect to the suspension frame. To address this
gap, Wang and Gong [91] simulated and analysed the suspension frame
under realistic load conditions representative of actual train operation.
Fig. 13 (a–c) illustrate the developed online monitoring system for
an HTS maglev train (a, b) and a representative measurement result
(c). The results demonstrate that the HTS maglev suspension frame
is capable of withstanding operational loads across diverse working
scenarios. Moreover, the frame’s strength and stiffness satisfy the re-
quired standards for safety, reliability, and performance. Inevitably,
irregularities in the permanent magnet track – whether random or
systematic – induce vibration responses in the suspension frame at
varying train speeds. Thus, vibration is an inherent characteristic of
HTS maglev systems. While condition monitoring systems have previ-
ously been developed for electromagnetic suspension (EMS) trains, no
comparable system had existed for the suspension frame of HTS maglev
trains. Given that the levitation height of the suspension frame directly
influences both suspension and traction performance, and that frame
vibrations affect dynamic behaviour and safety, a dedicated monitoring
system was implemented. This system measures lateral and vertical
vibration accelerations as well as levitation height at different train
<figure><img src="image_16.png" /><figcaption>Fig. 13. (a) Installation layout of the acceleration and levitation height sensors used to monitor levitation properties [91]. (b1) Signal acquisition system installed on the train. (b2) Levitation height acquisition system. (b3) Suspension frame vibration state acquisition system. Images (c1-c8) present typical measurement results for a maximum running speed of 0.5 m/s: (c1) Lateral vibration acceleration of the left-front suspension frame sensor: -0.13 m s⁻² to 0.14 m s⁻². (c2) Vertical vibration acceleration of the left-front suspension frame sensor: -0.12 to 0.11 m s⁻². (c3) Lateral vibration acceleration of the right-front suspension frame sensor: -0.28 to 0.23 m s⁻². (c4) Vertical vibration acceleration of the right-front suspension frame sensor: -0.17 to 0.19 m s⁻². (c5) Lateral vibration acceleration of the left-rear suspension frame sensor: -0.18 to 0.14 m s⁻². (c6) Vertical vibration acceleration of the left-rear suspension frame sensor: -0.25 to 0.25 m s⁻². (c7) Lateral vibration acceleration of the right-rear suspension frame sensor: -0.01 to 0.16 m s⁻². (c8) Vertical vibration acceleration of the right-rear suspension frame sensor: -0.19 to 0.18 m s⁻².</figcaption></figure>

Source: Reprinted with permission from Ref. [91].

speeds. The recorded maximum vibration accelerations and levitation height fluctuations remained within safe limits.

These findings validate the simulation model of Ref. [91], confirming that the suspension frame design is robust and that the HTS maglev train can operate safely under the investigated conditions.

To improve the utilization efficiency of the permanent magnet (PM) guideway and enhance the levitation force, a novel HTS maglev system employing a three-layer sandwich structure was proposed in Ref. [92]. In this design, HTS bulks are strategically arranged in three layers on both the upper and lower sides of the PM guideway (Fig. 14(a)), thereby increasing the effective amount of superconducting material and strengthening the levitation interaction. Based on the levitation force characteristics, a digital prototype of the proposed system was constructed using UM (Universal Mechanism) software, consisting of a carriage and six suspension frames (Fig. 14(b)). Fig. 14(c1-c6) present the simulated gap heights $h_z(t)$ as a function of time for six different operational speeds. These extended simulations enable detailed investigation of the prototypical behaviour of a complete HTS maglev train prior to physical prototyping. The dynamic response of the system was further analysed under varying operating speeds, providing a comprehensive evaluation of its running performance, levitation stability, and overall feasibility [92].

## 4.4. Summary of the maglev developments

To summarize this part, the bibliometric analysis of Section 2 has identified 38 articles dealing with maglev and railway. Of course, the literature on this topic is much larger. Performing a literature search in WoS with the topic "maglev" yields 2620 articles which also comprise the works on traditional magnetic maglev. Adding the search term "supercond*" reduces the amount of papers to 876, from which only 24 (= 2.7%) scored a SDGs tagging to SDGs 03 and 07.

This value is an extremely bad one as compared to the 25% obtained for the search "railway AND supercond*" (see Section 2). The reason for this result is the micro citation topic "Critical current density", which is given by WoS to most articles dealing with levitation as discussed in Ref. [38,39].

The articles we look at in this review apply superconductivity but also mention the term "railway", thus we expect to find here the articles dealing with properties concerning the operation of maglev trains, or details concerning easier and safer operation. Interestingly, the largest group of these articles [105-114] deal with HTS bulks and their levitation characteristics intended for superconducting flux-pinning maglev, and one article provides a useful comparison of the performance of superconducting coils versus superconducting bulks [114]. One could now think that these articles stem mainly from the earlier years, but this is not the case due to improved quality of the superconducting materials employed and the further development of numeric analysis tools. Practically all these papers make use of advanced numerical simulations to obtain the levitation characteristics (magnetic forces, torque and different lateral displacements as well as magnetic stiffness of various arrangements of the bulk pellets). This type of work will certainly continue and produce further important results.

The development of the flux-pinning maglev away from laboratory demonstrators and the people mover systems towards high-speed-like train systems will provide many new design and engineering issues to be solved, as can already be seen in the developments of CRRC shown in Fig. 10 (a–d). Thus, one can expect many interesting articles to appear in this direction in the near future.

We must note here that there is only 1 article concerning Hyperloop included in the search [81], which is an indicator that the Hyperloop project(s) are still in their infancy, whereas 2 articles focus on novel types of levitating vehicles and combined propulsion, levitation and guidance [92,115], and 3 articles are mainly concerned with electric effects like filters for motion control [116,117]. However, the remaining rest of the articles deal indeed with important issues of railway-like operation, concerning damping and beat vibrations [82,91,118-120], the construction of turnouts/switches, which are essential tools for a railway-like operation of the maglev systems, and the required guideways and the arrangements of the magnetic materials [121-125], but also with the development of novel magnet systems for superconducting maglev trains [126-128].

Superconducting maglev technology has already demonstrated its huge potential, with the JR Central Chuo Shinkansen Series L0 (improved version) seven-car train setting a world speed record of 603
<figure><img src="image_17.png" /><figcaption>**Fig. 14.** (a) Schematic diagram of a novel-type HTS maglev train [92]. Note the different design of the track as compared to Ref. [62]. (b) Digital prototype of the novel-type HTS maglev train. Images (c1-c6) present various time-history responses of the levitation gap, $h_z$, for six running speeds. Four vertical displacement and acceleration sensors are installed on each suspension frame, located at the left front (LF), left rear (LR), right front (RF), and right rear (RR) positions, respectively. (c1) 100 km/h, (c2) 200 km/h, (c3) 300 km/h, (c4) 400 km/h, (c5) 500 km/h, (c6) 600 km/h.</figcaption></figure>

*Source:* Reprinted with permission from Ref. [92].

km/h [60]. Commercial service on the Tokyo-Shinagawa – Nagoya line, now scheduled for 2034, will mark another milestone, while the integration of high-temperature superconducting coils in the new M10 middle car signals further progress. Meanwhile, HTS flux-pinning maglev offers a compelling research frontier, potentially underpinning future transportation concepts such as the Hyperloop.

# 5. Conventional electrified railways

## 5.1. History, distribution and speed records

Conventional electrified railways on the normal gauge (= 1435 mm) exist since 1895, when the first DC electrified railways opened (650 V, Meckenbeuren-Tettnang, Germany; 600 V, New York, New Haven & Hartford, USA; 650 V, Baltimore & Ohio, USA, 13-mile Hudson and East Rivers tunnel at New York, 1910), followed by 3-phase electrification (750 V, 40 Hz, Burgdorf-Thun, Switzerland 1899; 3 kV, 15 Hz, Valtellina railway, Italy 1902; 3 kV, 15 Hz, Simplon tunnel, Switzerland/Italy 1905) [129]. In 1905 began the first attempts to employ high-voltage AC current in Switzerland (Seebach-Wettingen, 15 kV, 15 Hz, later 16 2/3 Hz) [5], which were setting the base for the first large electrifications in Switzerland, Germany, Sweden, Norway, and Austria.

These countries finally agreed on a common style of electrification using 15 kV, 16 2/3 Hz in 1911 [130]. The DC electrification became popular in France (1.5 kV) from 1925, often as an extension of already existing third-rail electrification of suburban commuter lines, but also in the alpine area due to the smaller tunnel profiles possible [131–133]. Currently, the length of direct current lines under 1.5 kV is equal to 5904 km, or about 1/4th of the national railway electrification [134].

This was followed by the Netherlands, where the first AC electrification of the Hofpleinlijn in 1908 (10 kV, 25 Hz) was changed in 1926 to 1.5 kV DC and largely extended over the country. In the U.K., the then independent Southern Railway introduced a DC 660 V third-rail system for the lines south of London, whereas the electrification with 1.5 kV DC was adopted as a standard in 1921 [136], and by 1956, the AC 25 kV, 50 Hz system was introduced by British Rail. In the 1930s, the DC electrification using the higher voltage of 3 kV became feasible, which soon enabled large electrified networks in Italy, Belgium, Spain, Czechoslovakia, Poland, and the former Soviet Union, whereas the low DC voltages of 600/750 V are commonly used for tramways, subways, and local lines as the traction motors accept this voltage without the weight of an on-board transformer.

The latest development for railway electrification is the use of high-voltage transmission at 20–25 kV and an industrial frequency
<figure><img src="image_18.png" /><figcaption>**Fig. 15.** Railway electrification in Europe. Note the double-electrification character of France, where even natural extensions of the 1.5 kV DC were electrified with 25 kV, 50 Hz AC (e.g., Dole – Vallorbe, Marseille – Nice). In the UK, a large network of 750 V DC third-rail remains south of London. The Czech Republic and Slovakia are split into 3 kV DC and 25 kV, 50 Hz AC. File from Wikimedia Commons with additions.</figcaption></figure>

of 50 Hz, eliminating the need for a dedicated railway-owned power network. This decision was facilitated by the interim development of grid-controlled converters and commutatorless motors. Following the first attempts in Germany (1936 opening of the test line in the Höllental/Schwarzwald between Freiburg and Neustadt (Baden) operating at 20 kV and 50 Hz [130,137]), the SNCF further advanced this concept under the leadership of Louis Armand (president of the SNCF from 1955 to 1958).

This effort led to the electrification of the main line from Valenciennes to Thionville, adopting the slogan “the lightest overhead system for the heaviest trains” and capitalizing on improvements in power electronics [133].

In the US several types of electrification were tested, including the 3-phase electrification (first Cascade tunnel, Great Northern RR, Washington). However, only the DC system (Chicago, Milwaukee and St. Paul RR, 3 kV from 1917 onwards) and the AC system (New York, New Haven and Hartford RR, 11 kV, 25 Hz from 1907 onwards, Virginian RR, 11 kV, 50 Hz, 1922, 11 kV, Pennsylvania RR, 50/60 Hz, 1928) developed into larger networks [138,139], which, however, nowadays have been removed with the exception of the North-East corridor Boston – Washington, DC, currently operated by Amtrak.

In Japan, the narrow-gauge (1067 mm gauge), electrified railway network makes use of 3 kV DC, and the high-speed Shinkansen lines (normal gauge) employ the AC 25 kV, 50 Hz system. This requires that some Shinkansen services running to Akita and Yamagata be operated using dual-operation equipped trainsets.

As a result of the history outlined above, the map of Europe (see Fig. 15) is quite a colourful one, showing the use of five different electrification systems, with several countries having more than one

system in operation [140,141]. In France, all new equipment is typically operating at 1.5 kV and 25 kV, 50 Hz, except such of only regional use which leads to the blue/yellow striped colour. In contrast, all recently constructed high-speed lines in France, Belgium, the Netherlands, Spain, and Italy are operating under 25 kV, 50 Hz. Germany also relies for high-speed services on the standard system of 15 kV, 16 2/3 Hz. All European railway companies nowadays also have vehicles (locomotives and train sets, including also high-speed trains), which are capable to operate on up to four different systems to enable cross-border services, for the high-speed trains (e.g., connections Frankfurt/M – Paris) as well as for various freight services, e.g., the Betuwe line from Zevenaar to Rotterdam harbour (Kijfhoek).

High-speed trains represent a distinct segment of railway operations, requiring innovative solutions and often serving as a catalyst for technological advancement within the industry.

The quest for high speeds is nearly as old as the development of the electromotor itself. In 1903, the Research Association for High-Speed Electric Railways, St.E.S. set a world speed record in Germany using a 3-phase electrified track of a military railway between Jüterbog and Zossen (Fig. 16(a)) [5]. This record was held for almost 30 years until the Kruckenberg rail Zeppelin (nickname: “Schienenzeppelin”) reached a top speed of 230.2 km/h on 21.06.1931 (Fig. 16(b)). Although this railcar is not driven electrically, it demonstrated the advantages of carefully designed aerodynamics and low weight, which became the main issue for all subsequent attempts to break speed records [135]. Fig. 16 (c,d) present the French (SNCF) activities of 1955 to tackle the speed record [133].

Two different engines, CC 7107 by Alstom (formerly Alsthom) and BB 9004 by Jeumont-Schneider, were used for the test on the track “Les Landes”, between Bordeaux and Dax, which was specially prepared. For
<figure><img src="image_19.png" /><figcaption>**Fig. 16.** (a) AEG 3-phase railcar developed by the Research Association for High-speed electric railways, St.E.S., reached a top speed of 210.2 km/h on 28.10.1903 (Image public domain). (b) Kruckenberg rail Zeppelin driven by a BMW 12-cylinder 450 kW motor powering a pusher propeller [135], reached a top speed of 230.2 km/h on 21.06.1931 (Photo by Franz Jansen (†), Erkrath; Germany, licence CC BY-SA 3.0 (26.6.1931)). (c) SNCF CC 7107, Alst(h)om, top speed 326 km/h, 28.03.1955. Cité du train, Mulhouse, France. (d) SNCF BB 9004, Jeumont-Schneider, top speed 331 km/h, 29.03.1955. Cité du train, Mulhouse, France. Inset: Commemorative plate, fixed on both engines. (e) SNCF TGV 16 Sud-Est, top speed 381 km/h, 26.02.1981, seen here in regular service at Geneva, Switzerland. (f) DB ICE 410 001, top speed 406.9 km/h, 01.05.1988, seen here on the high-speed section at Graben-Neudorf, Germany. (g) SNCF TGV 325 Atlantique, top speed 515.3 km/h on 18.05.1990. Seen here in regular service in Geneva, Switzerland. (h) SNCF TGV V150 with the top speed of 574.8 km/h on the LGV Est between the stations Meuse and Lorraine, France, 03.04.2007, Photo A. Stoll, licence CC BY-SA 2.0. All other photographs by MRK.</figcaption></figure>
engines, the transmission was changed, and engines and cars received special aerodynamic treatment [142]. The CC 7107 reached a speed of 326 km/h, but was badly damaged afterwards. The BB 9004 reached a top speed of 331 km/h, but on the price of damaged track and catenary, so no more runs were possible. Thus, both engines were considered as equal.

As no additional runs were possible, both machines received a commemorative plate as shown between Fig. 16 (c,d). Here, it must be noted that this record was achieved on a line electrified by 1.5 kV DC, but with increased voltage and special catenary. Again, this record lasted for quite a long time until it was pushed up to 381 km/h on 26.02.1981 by a new TGV train set (see Fig. 16(e)). Figure (e) presents the TGV trainset no. 16, which set the record to 381 km/h, in regular service at Geneva station in Switzerland, which has a section electrified with 1.5 kV DC.

In (f), the ICE Experimental (ICE prototype series 410) is shown in the high-speed section at Graben-Neudorf, Germany. This trainset improved the high-speed record to 406.9 km/h. In image (g), a TGV Atlantique is shown that set the record to 515.3 km/h, directly competing with the JR maglev. The photo was taken when the train made regular service at Geneva station, Switzerland, wearing a banderole indicating the 515 km/h. Finally, image (h) presents the TGV V150 which reached a top speed of 574.8 km/h on the LGV Est between the stations of Meuse and Lorraine. The analysis of Section 2 found 78 articles in the search dealing with applications of superconductivity in existing, electrified railway systems. Most of these articles deal with superconducting feeder cables, including the modelling, design, and setup of prototype systems [143–165]. This topic dominates the current ongoing research efforts, where especially in France and Japan such superconducting feeder cable units are being built for daily use. Two important issues to be solved are the development of suitable joints for the feeder cables [19,166–169] and their terminations [170]. This large group of articles, which makes up in total 29 articles (corresponding to 37.1% of articles on conventional, electrified railways), also forms the current main direction of the research published.

Another very important research direction is the wireless power transmission to vehicles using HTS technology [171–174,174–177]. 7 articles deal with this issue, which is important for tramways, where overhead wires are often not wanted by the authorities, but also for recharging battery-electric cars as well as for high-speed trains to avoid problems with the catenary. Furthermore, in Section 4 we have seen that JR Central now uses wireless power transmission to their maglev trains [60], which cannot operate with a catenary.

Then, a large number of articles deal with the improvement of the supporting grid for the railway systems. The use of SMES in the railway environment is considered in Refs. [32,178–182], and the use of FCL in the railway environment is discussed in [35,183–192], which in total comprises 17 articles (corresponding to 21.8% of articles regarding conventional and electrified railways).

Applications of superconductivity within the railway rolling stock are discussed in 20 articles (corresponding to 25.6%), although one could expect here a much larger number of articles with respect to the ongoing development of high-speed trains and the pressure towards operating speeds of ~300 km/h. Interestingly enough, a recent article deals with superconductor linear eddy current brakes for high-speed trains [193], which shows that this topic is considered worthwhile research.

In earlier years, significant attention was devoted to the development of superconducting motors [195–202] and superconducting transformers [203–210] for railway rolling stock. However, this line of research has recently lost momentum. The main reason lies in the fact that present-day conventional electric motors already demonstrate high efficiency, robust performance, and relatively low weight. Consequently, there is currently little demand to pursue further weight reduction as a means of optimizing high-speed railway operation, as also discussed in Refs. [200,201].

The most recent articles in this direction present new developments for lightweight urban rail transit vehicles [195,196], especially to increase the efficiency of linear induction motors. The authors propose a different concept, adopting HTS coils to replace the otherwise required permanent magnets to widen the otherwise existing narrow speed range of such train systems.

The electrification of rail transport demands efficient and lightweight power components. Conventional railway transformers are characterized by substantial weight (typically ~15% of the total weight of the locomotive) and require large volumes of ester for cooling, which make up ~2 tons. These constraints provide strong motivation for the development of superconducting transformer technology, which promises significant reductions in mass and improved operational efficiency. Between 2000 and 2005, major research efforts were undertaken by two groups, namely the Railway Technical Research Institute (RTRI, Japan) and Siemens (Germany). Their projects employed Bi-2223 superconducting tapes as the active conductor material for prototype superconducting transformers. While these studies demonstrated the feasibility of large-scale applications of high-temperature superconductors (HTS), challenges such as cryogenic stability, AC loss management, and mechanical robustness of the superconducting tapes remained limiting factors.

In addition to applied developments, supporting studies have addressed both electrical considerations [211–213] and fundamental investigations of superconducting materials [34,214,215], with particular emphasis on the stringent requirements of railway operations. Together, these efforts complete the scope of research activities in the field of superconductivity for railway applications.

## 5.2. Applications in railway vehicles

### 5.2.1. Superconducting transformers

The class 185 represents a multipurpose, dual-frequency (16 2/3 and 50 Hz), dual voltage (15 kV and 25 kV) unit, intended for cross-border service.

Thus, the engines perform service mainly between France and Germany. The main parameters of this locomotive are: Total weight 82 t, average axle load 20.5 t, maximum speed 140 km/h, maximum tractive effort starting at 300 kN, Adtranz 2000/Bombardier design [194]. The transformer unit (marked red in Fig. 17) has a weight of 12,000 kg, including the ester filling as a coolant with a weight of 2000 kg, nominal capacity 5400 kVA. The weight of the built-in electric motors is 2050 kg (three-phase asynchronous motors, marked in blue in Fig. 17), the voltage is 1870 V, and the maximum rotation speed is 3320 rpm [194]. The engines of this type belong mostly to the German Railways Cargo, but also to private open-access operators registered in Germany, including SNCF-Akiem.

Research for superconducting transformers for rolling stock was carried out in Japan and Germany. In Refs. [204–207,210], a lightweight and highly efficient superconducting traction transformer was developed. The intended use of this transformer was Shinkansen rolling stock, where the transformer would be mounted within the car body. Its primary voltage is 25 kV, which is commonly employed as catenary voltage on high-speed railway lines around the world. RTRI developed a prototype transformer with a primary winding, four secondary windings, and a tertiary winding, all using Bi-2223 superconducting tape as depicted in Fig. 18. The resulting overall weight of the demonstrator was 1.71 t, excluding the refrigerator and compressor. Liquid nitrogen cooling for the transformer unit was adopted. The maximum output available to maintain superconductivity is 3.5 MVA.

In Refs. [208,209], the approach from Siemens, Erlangen, Germany is described. This work took place in the years 1998–2001, when a large number of engines was ordered by the German Railways. With the introduction of HTS technology, transformer units can now be manufactured to be significantly lighter and more compact – up to
<figure><img src="image_20.png" /><figcaption>**Fig. 17.** (a) Sketch of a multi-purpose, bi-frequency (15 kV, 16 2/3 Hz and 25 kV, 50 Hz) AC locomotive series 185 of the German railways (DB), intended mainly for cross-border freight service [194]. The red arrow indicates the location of the underfloor transformer, the blue arrow points to the location of the four electric motors. (b) Photograph of Akiem-SNCF 185 557-6 in Mannheim Hbf. (c) Photograph of DB 185 025-4 in Saarbrücken Jägersfreude. Photographs by MRK.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Fig. 18.** Superconducting traction transformers. (a1): RTRI transformer for Shinkansen rolling stock. (a2): Scheme. Reproduced with permission from Ref. [204]. (b1): 1 MVA superconducting transformer designed by Siemens laboratories, Erlangen. (b2): Transformer mounted in a holding grid to fit in the engine like shown in Fig. 17(c). (b3): Test of the transformer unit in the laboratory. Images (b1-b3) provided by Siemens [216].</figcaption></figure>

55% smaller than their conventional counterparts – while also achieving efficiencies of approximately ~99%. This improvement directly translates into substantial energy savings and reduced environmental impact, aligning with the objectives of the Sustainable Development Goals (SDGs).

Based on conceptual HTS designs for various classes of German Railway rolling stock – including the regional train class 424, the high-speed train ICE 3, as well as the locomotive BR 152 (freight-only version, similar to the series 185 shown in Fig. 17) drive cycle simulations were performed for selected reference routes [209]. The authors found that the energy losses associated with conventional copper-and-oil transformers, including transformer and cooling losses, range from 0.5 kWh/km to 2.6 kWh/km, being clearly higher than those of HTS-based transformers. Thus, the energy-saving potential of implementing HTS transformers ranges from approximately 200 MWh to 1600 MWh per vehicle per year, depending on factors such as vehicle type, route characteristics, and operational conditions [209]. Based on these findings, a first demonstrator unit – a 1 MVA transformer cooled with subcooled liquid nitrogen (T = 67 K) – was developed [208]. In addition to improving energy efficiency, the HTS transformer demonstrator demonstrated several operational benefits, including a 30%–50% reduction in weight, enabling higher payload capacity or improved acceleration profiles, and significantly lower noise emissions compared to conventional copper-and-oil designs. Furthermore, its compact size simplifies vehicle integration and creates additional space for auxiliary onboard systems, while the cryogenic cooling system provides inherently low acoustic noise and minimal vibration, supporting enhanced passenger comfort and reduced environmental impact. As superconducting material, Bi-2223 tape from NST A/S and Vacuumschmelze (now Bruker BEST) was chosen, from which 6.8 km of Bi-2223 were incorporated into the demonstrator [216]. The dimensions of the transformer are 88 × 108 cm², allowed for seamless integration into the available engine space, as illustrated in Fig. 17. The efficiency η of this transformer was recorded to 97.8% (compared

to η of conventional transformers of approximately 92%–95% [208]). These promising results provided a strong foundation for scaling up the technology and preparing a full-size system for in-engine testing [208].

### 5.2.2. Superconducting eddy current brakes

Braking systems are a critical aspect of railway system design, particularly in high-speed rail applications. Traditionally, adhesion braking, which relies on the adhesion coefficient between the wheel and the rail, has been the primary method used in train braking systems. However, the effectiveness of adhesion braking is highly dependent on the condition of the rail surface. Furthermore, frictional contact between braking components and the rail can lead to wear and damage, resulting in increased maintenance costs. An alternative to adhesion-based braking is the linear eddy current brake (LECB), a type of non-adhesion braking system [217,218]. This technology has been implemented in trains such as the German ICE 3. The LECB systems are typically excited using permanent magnets, copper coils with yokes, or a combination of both. Thus, these systems require an air gap of less than 10 mm between the brake poles and the rail. However, their considerable weight and constrained installation locations add a significant load to the bogies of the train. The use of superconducting coils can offer a promising solution due to their ability to generate large excitation currents and to produce high magnetic induction intensities. One key advantage is that the air gap between the magnetic poles and the rail can be significantly larger compared to that of systems that use permanent magnets or copper coils. This feature potentially eliminates the need to mount the braking system directly on the train bogie. In fact, the research on the application of superconducting conductors in braking systems remains scarce and largely unexplored, so only one recent paper [193] deals with this aspect. In this work, two-dimensional (2-D) and three-dimensional (3-D) analyses of the eddy current brake system were conducted using ANSYS software. The results of both simulations demonstrated that a magnetic induction intensity and current of 180 A are sufficient to meet
the operational requirements of a superconducting wire. In the 2-D analysis, the relationship between various parameters and the resulting braking force was investigated. These parameters included the number of coils, the distance between the magnetic poles, and the air gap between the coils and the rail surface. For the 3-D model, train speeds of 28 m/s and 110 m/s were simulated to evaluate performance under different operating conditions. The results obtained are promising and indicate the feasibility of using superconducting technology in eddy current braking systems. These findings are expected to encourage further research and development in this area in the near future.

### 5.2.3. Superconducting motors

Among the papers dealing with the development of HTS motors, there is only one paper [202] considering the replacement of existing DC motors for Japanese railways, operating with 3 kV DC. Induction motors have been widely adopted in place of traditional DC motors for DC electric rolling stock due to their superior efficiency and utility. However, further improvement in the motor performance is essential to enable the development of a more environmentally friendly DC railway system in the future. To achieve this, it is important to explore more efficient machine designs, particularly for next-generation high-performance DC electric rolling stock. In this context, a fundamental study aimed at reducing energy consumption in electrified railway systems through the application of HTS technology was undertaken in Ref. [202]. As an initial step, the specifications for an HTS induction/synchronous motor (HTS-ISM), which is the target HTS motor for the aforementioned application, were defined. Additionally, the relationship between the current-carrying properties of the HTS tape conductors and the torque characteristics of the HTS-ISM were investigated. The results obtained demonstrate the potential of HTS-ISM as a core component in the realization of next-generation DC electrified railway systems.

All other papers identified in the literature search [195-201] focus on the design and characteristics of linear HTS motors, which propose a concept similar to that of the JR Maglev drive, but adapted for conventional railways. While this concept is technically intriguing, its practical implementation poses significant challenges, particularly with regard to integration into existing conventional railway infrastructure. As a result, the application of such linear HTS motors is considered more viable for lightweight urban transit systems [195,196] or subway networks that operate independently of the main railway system, where integration constraints are less severe.

### 5.2.4. Superconducting wireless energy transfer

Wireless energy transfer (WET) for railway vehicles is an emerging and increasingly critical area of research, driven by the need for low-maintenance, sustainable and efficient power supply solutions in modern rail systems [219-221]. As the railway industry pursues advancements such as storage battery-powered trains, autonomous operations, and smart infrastructure, traditional wired power systems – such as those using overhead lines and pantographs – are becoming increasingly limiting due to mechanical wear, maintenance demands, and infrastructure complexity. WET offers a promising alternative by enabling contactless power delivery, potentially reducing system degradation, improving reliability, and enabling new forms of electrification and charging for urban, regional, and high-speed rail applications. Consequently, ongoing research is focused on developing high-capacity, efficient, and low-loss wireless power transmission systems, particularly those that leverage HTS technologies to address performance and scalability challenges.

Among the new developments, storage battery vehicles have been explored as a promising solution to reduce greenhouse gas emissions in railway transportation. These vehicles are typically charged through wired power transmission systems, utilizing overhead wires and pantographs. However, this conventional charging method presents several challenges, including mechanical wear, electrical discharge, and the

risk of disconnection between the overhead lines and pantograph components. To address these issues, research has shifted towards wireless power transmission (WPT) systems capable of delivering several hundred kilowatts of power to railway vehicles. Despite their potential, conventional large-capacity WPT systems face limitations, particularly due to heat generation in the copper coils, leading to a temperature rise. In addition, to mitigate switching losses in power converters and eddy current losses within the coils, the operating frequency of the system must be reduced to below 10 kHz. In response, research was undertaken mainly at Okayama University, Japan, to investigate a high-capacity low-frequency WPT system for stationary railway vehicles, using HTS coils [171-177]. While several studies have examined low-capacity or low-frequency WPT systems using HTS technology, one of the key remaining challenges is the reduction of AC losses in HTS coils, which tend to increase with higher transport currents. With the idea to minimize these losses, it is essential to determine the optimal resonance circuit topology – namely Series-Series (SS), Series-Parallel (SP), Parallel-Series (PS), and Parallel-Parallel (PP) configurations. However, comprehensive design guidelines for selecting appropriate topologies in large-capacity WPT systems using HTS coils are not yet well established. Thus, the most recent paper [183] performed a finite element method (FEM) analysis to evaluate the power transmission characteristics of various resonance circuit topologies in WPT systems employing HTS coils. Additionally, design guidelines tailored for the implementation of these topologies in railway vehicle charging systems were worked out. Thus, the advantages in this field may pave the way for more superconductivity-based applications in the railway sector.

## 5.3. Infrastructure of electrified railways

### 5.3.1. Superconducting feeder cables

Fig. 19 summarizes data from several recent superconducting cable trials aimed at evaluating their reliability, primarily in power grids and, more recently, in traction networks [55,222-228]. To the best of our knowledge, Japan was the first country to implement a superconducting cable in a railway network. This effort began with a 31-meter-long prototype tested at 10 V, followed by the installation of a 310-meter-long superconducting cable in Tokyo in 2016. This bipolar cable, designed for a nominal current of 1 kA, comprises two types of superconducting tape: 78% Bi-2223 tapes and 22% YBCO tapes [150]. Many DC electrified sections in Japan as well as in Europe suffer from significant power losses and voltage drops, similar to the issues faced by subway systems in the United States and by high-speed TGV trains in France operating on DC electrified sections. Such runs are often hundreds of kilometers long (e.g., TGV services Paris - Geneva (Switzerland), Paris - Torino (Italy)), not just passing the suburbs to reach the final stations. Consequently, superconducting power transmission has emerged as a promising solution for addressing these challenges and enhancing the operational stability of trains [229].

Recent analysis [146] revealed that about 5% of the electricity is lost during transmission along the railway lines, representing a substantial inefficiency observed across rail networks worldwide. Given the pivotal role of power transmission technology in railway systems, reducing these losses is a key step towards improving energy efficiency and supporting sustainable railway operations. Ongoing projects such as SuperRail and similar HTS-based traction initiatives are expected to further demonstrate the benefits of superconducting technologies, paving the way for their broader adoption in future high-efficiency railway networks.

#### Superconducting feeder cables in Japan

In Japan, the Railway Technical Research Institute (RTRI) launched a project to develop technologies aimed to reduce power transmission losses in next-generation railway systems through the use of superconducting cables. The outcomes of this initiative have been documented in several publications [146,147,149,150,153-160,162,164,165]. The newly developed superconducting feeder cables were tested on both
<figure><img src="image_22.png" /><figcaption>**Fig. 19.** Summary of recent global projects involving superconducting DC cables. The figure plots the length of superconducting cables used in various (railway) applications versus their respective operating voltages.</figcaption></figure>

*Source:* Reproduced with permission from Hajiri et al. [148].

conventional, electrified railway lines within the JR network (see the illustrations and sketches in Fig. 20 (a–d), [158]) as well as on a subway line with third rail power supply near the Nakanofujimicho substation on the Tokyo Metro Marunouchi Line as shown in Fig. 21 (a–c2). This work was originally reported in Refs. [150,157], where the Railway Technical Research Institute (RTRI) established the fundamental characteristics of long superconducting feeder cables during system cool-down. The study also identified the primary mechanical and thermal challenges arising in this process and proposed several practical solutions, many of which have since been refined and validated in subsequent studies.

Railway systems utilizing a third rail configuration require a high current supply due to the relatively low operating voltage, which is commonly 600 – 750 V [150]. In such systems, the application of superconducting cables is particularly beneficial, as it helps to minimize voltage drops and to reduce the overall energy consumption. As part of the project, a superconducting cable was installed on an operational railway line with a third rail system to perform both cooling and current load tests, thus demonstrating its feasibility and performance in real-world conditions.

When constructing long superconducting cables along railway lines, heat-shrink forces induced during cool-down must be carefully managed. First, a suitable installation method was developed for railway applications. Second, a 300-m-class cable was installed on a test track, and its displacement during cooling was measured in detail. Third, an X-ray radiographic inspection was performed over the full cable length to verify the suitability of the laying method and detect defects such as buckling, which can cause contact between the cable and cryostat, increasing thermal losses.

In this trial, the cable layout was designed to replicate actual railway conditions, including alignment along the track and passage through road and track crossings. As a result, the route incorporated multiple curves to absorb thermal contraction, along with an offset section, and a movable current-terminal system to accommodate shrinkage. Fig. 6 shows the completed installation, with a total length of 310 m between the east- and west-side current terminals. The cable was installed successfully using the prescribed feed-out method [150].

Stress relief for long superconducting cables was then analysed in detail [150]. During cooling from room temperature to liquid nitrogen temperature, the cable former and insulating pipe—both made from 304 stainless steel with a thermal expansion coefficient of

17.3 × 10⁻⁶/°C—shrink by approximately 0.3%, corresponding to 0.9 m for a 300-m cable. Thermal contraction of the superconducting tapes themselves was neglected, as they were wound slightly loosely in a spiral pattern. Without mitigation, such shrinkage can impose mechanical stress on the cable and terminals, potentially causing damage. To address this, three countermeasures for heat-shrink compensation were considered.

Three countermeasures for heat-shrink compensation were evaluated:

(i) Movable current terminal — The terminal is mounted on a wheel-based platform to allow movement in response to contraction. In this trial, movement was limited to ~0.5 m by site constraints, making this approach unsuitable for kilometer-class installations.

(ii) Snaked cable configuration — The cable is laid in a controlled “snake” pattern, with amplitude and wavelength matched to the expected contraction. During cool-down, the snaking straightens, absorbing shrinkage without imposing stress.

(iii) Offset sections — Arc-shaped offset sections are introduced along the cable route. These arcs flex and straighten during cooling, accommodating the shrinkage while maintaining alignment.

The longitudinal displacement of the cable during cooling was measured in detail [150]. Notably, some displacement was observed prior to cooling, attributed to evacuation of the insulating pipe via vacuum pumping.

After cooling, displacement occurred along the entire cable length, with the direction of displacement reversing between the left and right sides of the rail and underground crossing points, which acted as fixed anchors. Fig. 9 presents the radial displacement at each lying point. At the offset point, the longitudinal displacement was effectively zero due to cleat fixation; however, a radial displacement of 27 mm was measured. The initial design premise was that the offset point would absorb cable displacement during cooling. In practice, however, displacement at the offset point was minimal because most of the thermal contraction was absorbed by the snake points and simple offset configurations.

The study of RTRI [150] confirmed further that both snaking and offset-section methods offer scalable, practical solutions for mitigating thermal contraction effects in long superconducting railway cables. Their adoption in future kilometer-class feeder systems is expected to enhance mechanical robustness, maintain cryogenic integrity, and ultimately improve the operational reliability of superconducting power transmission in high-speed rail networks.

To evaluate practical installation methods for long superconducting feeder cables in railway applications, different laying configurations were considered [150]. A design combining one snake point and one offset point was selected for experimental verification. A 300-m-class superconducting feeder cable was installed on a test track replicating actual railway conditions, including road and rail crossings. Following installation, the cable was gradually cooled over 80 h by controlling the cold gas temperature via an evaporator, thereby avoiding the application of sudden thermal stress. Details of this configuration are illustrated in Fig. 22. After the set up was completed, the cable was cooled gradually by controlling the cold gas temperature with an evaporator so that the cooling stress might not be suddenly applied to the cable. Initial cooling has been completed in 80 h. As a result of an examination the cable after cooling, it was found that there were no traces of large displacement as well as excessive stress.

The longitudinal displacement of the cable was monitored throughout the cooling process [150]. Some movement occurred even before cooling, caused by evacuation of the insulating pipe with a vacuum pump. After cooling, displacement was observed along the entire cable length, with reversal of displacement direction between the left and right sides of the rail and underground crossing points, which acted as fixed anchors. Fig. 22 shows the radial displacement at each laying point. At the offset point, the longitudinal displacement was effectively zero due to cleat fixation; however, a radial displacement of 27 mm
<figure><img src="image_23.png" /><figcaption>**Fig. 20.** Development at RTRI of superconducting feeder cables, tested on the JR network. (a) The superconducting feeder cable installed parallel to the tracks. (b) Train running test using superconducting feeder cable on a commercial railway line, showing also the substation. (c) Train passing the substation with the installed superconducting feeder cable. Images provided by M. Tomita (RTRI). (d) Superconducting feeder cable system set up along the test track on the premises of (RTRI). The inset (blue box) gives the details of the superconducting wire.
*Source:* Reproduced with permission from Ref. [158].</figcaption></figure>

was recorded. Although the design premise was that the offset point would absorb most of the contraction, measurements indicated that the majority of the displacement was actually taken up by the snake section and simple offset sections.

Fig. 23(c) presents the measured vertical displacement of the cable at the offset point. The results show that the cable core was displaced in opposite directions on either side of the central cleat, causing contact with the insulating inner pipe. This contact can generate localized mechanical forces, reinforcing the need for stress-relief features.

Post-cooling inspection confirmed that there were no signs of large-scale displacement or excessive mechanical stress [150]. X-ray radiography of the entire cable length as illustrated in Fig. 23(d) verified the absence of buckling or rupture, indicating that the laying method was mechanically sound. Internal X-ray imaging at the offset point distinguished the cable core, insulating inner pipe, and outer pipe, and revealed contact between the inner and outer pipes. Such contact is known to increase heat ingress—previous measurements indicate a 1.2× increase under these conditions [160].

This finding highlights the importance of incorporating design features that suppress thermal invasion even in cases where structural contact occurs.

Overall, the results obtained in Ref. [150] demonstrate that combining snake and offset sections provides an effective stress-relief strategy for thermal contraction in long superconducting feeder cables. The approach maintains mechanical integrity during cool-down and minimizes the risk of structural deformation, making it suitable for kilometer-class installations in railway power systems. Looking ahead, such installation methodologies could accelerate the adoption of superconducting feeder technology in both conventional high-speed rail and maglev systems, enabling higher current capacity, reduced transmission losses, and improved energy efficiency—contributing directly to emission reduction goals in future rail transport.

This study confirmed that both snaking and offset-section methods offer scalable, practical solutions for mitigating thermal contraction effects in long superconducting railway cables. Their adoption in future kilometer-class feeder systems is expected to enhance mechanical

robustness, maintain cryogenic integrity, and ultimately improve the operational reliability of superconducting power transmission in high-speed rail networks. From a maintenance perspective, incorporating periodic X-ray or other non-destructive inspections into the operational schedule will be essential for detecting structural shifts, insulation contact, or deformation before they impact performance. Combining such monitoring with predictive maintenance algorithms and real-time thermal performance tracking could further extend service life, reduce unplanned outages, and ensure the long-term viability of superconducting feeder systems in large-scale railway operations.

The ongoing development of superconducting feeder cables forced the solution of another problem. HTS DC feeder cables have been proposed as a means to reduce energy losses in railway power systems, for normal trains, subways, and tram-train operations. However, for the required tests, these cables must be installed during short night-time maintenance windows, necessitating the use of a quickly-fabricable, low-resistance joint [19,166–169]. Traditionally, soldering techniques have been widely adopted for connecting round-shaped HTS cables in both transmission and feeder systems. The RTRI proposed, however, an alternative jointing method involving press welding with indium, which was done either at room temperature (using cold pressure welding) or with a heat treatment applied in the range of 90-120 °C. This technique offers potential advantages in both speed and reliability for cable-to-cable connections in railway applications.

In summary, the research conducted by the Railway Technical Research Institute (RTRI) on superconducting feeder cables, as reported in Ref. [150,158], has established the key mechanical and thermal behaviours of long cable systems under cryogenic operation. The work identified the principal challenges associated with thermal contraction, and demonstrated effective installation methods – such as snake and offset configurations – that mitigate cooling-induced stress while preserving electrical performance.

### Superconducting feeder cables in France

In France, the deployment of a superconducting feeder cable is currently being evaluated by SNCF within the framework of the SuperRail project. The initiative focuses on installing two 60-meter-long
<figure><img src="image_24.png" /><figcaption>**Fig. 21.** RTRI superconducting feeder cable for third-rail subway system. (a) Schematic sketches of the catenary power supply and the third-rail power supply used for subway lines. (b1) shows the feeding circuit of the third-rail system incorporating a superconducting feeder cable installed in parallel with the existing feeder. During operation, power from the substation is distributed between the conventional feeder and the superconducting cable. (b2) presents the results of the superconducting feeder cable conduction test. The inset provides a schematic sketch of the HTS cable. (c1) shows an example from the electric railcar running test, recorded at a sampling rate of 0.1 s. The first tier indicates train velocity, from which acceleration was derived. The second tier shows the voltage applied to the train, the third tier the current drawn by the train, and the fourth tier the current flowing through the superconducting feeder cable. During traction, current flow was accompanied by a voltage drop; during braking, the current reversed direction, the voltage increased, and regenerative power transfer was confirmed. The fifth tier shows the voltage between the superconducting feeder cables. (c2) gives a recording of the temperatures of the refrigerator cold head and the current-lead terminal, which remained stable throughout the train running test, confirming effective cryogenic performance under operational conditions.</figcaption></figure>

*Source:* Reproduced with permission from Ref. [147].

superconducting cables on a commercial railway line between the electrical substation in Vouillé and the catenaries supplying power to Montparnasse Station in Paris [230]. This section of the network operates under a 1.5 kV DC electrification scheme. Montparnasse Station, one of France's major transportation hubs, served 50 million passengers in 2020, and forecasts project that this figure will reach 90 million by 2030. In light of these increasing demands, improving the robustness and reliability of the station's electrical supply has become a key priority, and superconducting technology offers a potential solution for addressing these challenges. In addition to improving supply reliability, superconducting cables have the potential to significantly reduce energy losses and voltage drops compared to conventional copper-based feeders, as demonstrated by recent techno-economic studies. These benefits can help railway operators improve operational efficiency, lower operating costs, and support the transition towards more sustainable and resilient railway infrastructure.

However, traditional reinforcement methods – such as installing additional copper feeder cables – are not feasible in this area due to severe right-of-way restrictions and limited available space, as illustrated in Fig. 24 (a–e). As a result, the project has adopted an innovative solution: the deployment of superconducting feeder cables, which offer higher power density and reduced losses in a compact form factor. SNCF issued challenging specifications for the project:

(i) Two electrically independent cables to supply each 1500 A/1.5 kV DC in rated conditions (max 3% of harmonics below 5 kHz) and 3500 A/1.5 kV DC current inrush which happens when trains accelerate to reach traffic speed. The return currents are flowing via the rails, connected to the negative (0) pole of the DC supplies.

(ii) Substation directly connected to the RTE transmission network at 63 kV, fault power of 100 MVA, fault current of 67 kA during 100 ms.

(iii) The cooling system requires high reliability as well as high efficiency, variable cooling capacity. Available cooling power of up to 1.2 kW at $T = 67$ K.

As part of this initiative, preliminary studies have been conducted [143,145,148], utilizing advanced modelling techniques to evaluate the performance of superconducting cables within the traction power network and to assess their potential impacts on the overall system behaviour. The project involved building a 3D digital twin of the area in order to virtually integrate the equipment, anticipate possible issues, and confirm the designs of the components to be installed.

Special considerations were required for the cooling system, which was installed on a skid at the substation (SS), to ensure sufficient circulation for both cooling and sub-cooling of $LN_2$. For use during type testing and final installation, an $LN_2$ circulation, pressurization,
<figure><img src="image_25.png" /><figcaption>**Fig. 22.** Superconducting feeder cables contract significantly when cooled from room temperature to liquid-nitrogen temperatures. For long installations in railway lines, this thermal contraction imposes mechanical stresses that must be addressed through carefully engineered countermeasures. The top figure illustrates the entire situation with the points ① – ④ shown in detail below. The image on the right presents the situation on the ground with the superconducting feeder cable paralleling the railway track in a duct.</figcaption></figure>

Source: Reproduced with permission from Ref. [150].

and cool-down management system was implemented, which required the installation of an LN₂ storage tank. The cooling is based on an LN₂ bath pumped within the cryostat, which is equipped with a heat exchanger for cooling the cable. Alternatively, a cryocooler can be connected in place of the pumped LN₂ cryostat to produce subcooled liquid nitrogen. A new family of cooling systems (called Reverse Turbo-Brayton, abbreviated as RTB [231]) was developed to cover – with a higher efficiency and low maintenance requirements – a range of cooling power of few kW at the temperature of LN₂ needed for urban superconducting systems. The RTB allows the cooling capacity to be controlled from 30% to 100% of the maximum power. A 1.7 kW @ 67 K RTB will be connected to the cooling skid instead of the pumped bath for long-term commissioning [145]. A schematic view of the operation of the feeder cable cooling system using the RTB unit is depicted in Fig. 25.

The work involved the development of advanced simulation tools for superconducting cables operating under railway conditions. These tools include transient electrothermal finite element (FE) simulations coupled with an electrical network circuit model to accurately represent cable behaviour within a traction power system. The feasibility of integrating new functionalities, such as current limitations, was also investigated. Additionally, a dedicated test platform was developed to validate numerical models and to perform high-current tests on superconducting cable prototypes under realistic operational conditions.

A 35 m superconducting cable test loop with a junction was constructed to carry out qualification and performance validation. The tests were performed in accordance with IEC 63075, including the following:

* Bending test to assess mechanical flexibility under installation conditions.

* Pressure test to assess structural integrity under operational pressure levels.

* Thermal cycling test to simulate operational temperature variations over an estimated 40-year lifespan.

* Critical current test to verify the superconducting performance of the cable.

In addition, tests were performed in compliance with EN 50124-1, which included (i) Lightning impulse withstand test to evaluate overvoltage resilience, and (ii) Dielectric test to validate insulation performance under high-voltage stress [148]. Finally, fault current tests were carried out to replicate operational fault conditions. Fig. 26(a) presents the experimental arrangement of the DC feeding cable and the cooling system at the SNCF Railway Test Agency (SNCF-AEF), and (b) shows a schematic sketch of the layout. Due to limitations at the test facility (maximum 40 kA at 1.5 kV), an equivalent energy fault test was implemented, simulating an energy release equivalent to 67 kA for 100 ms.

In Ref. [148], the electromagnetic performance of two HTS cable topologies – unipolar and bipolar – was analysed in detail, with a focus on their impact on a direct current (DC) railway network under load conditions. A detailed finite element (FE) model was developed using COMSOL Multiphysics, incorporating the non-linear dependence of the superconducting cable’s electrical resistivity, expressed as $\rho(J, B, T)$, where J denotes the current density, B is the magnetic flux density and T is the temperature. This FE model was integrated with a lumped-parameter circuit model of the railway network, which is particularly
<figure><img src="image_26.png" /><figcaption>**Fig. 23.** (a) Heat-shrink mitigation strategies for long superconducting feeder cables, illustrating snake and offset configurations designed to absorb thermal contraction during cool-down. (b) Radial displacement profile measured at each laying point, showing localized movement concentrated at snake sections rather than offset points. (c) Vertical displacement at the offset point, quantified as the offset between the centres of the cable core and insulating pipe, indicating minor misalignment due to thermal contraction. (d) X-ray radiograph of a 10 m offset section, revealing direct contact between the insulating inner and outer pipes—a condition known to increase thermal ingress by ~20%—underscoring the need for contact-suppression measures in future designs.
*Source:* Reproduced with permission from Ref. [150].</figcaption></figure>

well-suited for transient simulations that account for train motion and dynamic loading [148]. Using a case study based on a section of the Parisian railway network, the analysis revealed that implementing a 5-km superconducting feeder cable could reduce electrical losses by up to 60% compared to conventional copper cables. Furthermore, replacing the feeder cables with superconducting ones led to a 8.6% decrease in the total electrical energy consumption of the traction power network.

In a 1.5 kV low-voltage DC grid, the distance between the substations (SS) is on the order of a few tens of kilometers [141], and the section of the overhead conductors can be as large as 1000 mm² [229]. In the field of railway electrification, SS are the sources of electrical power supplied to the traction lines. In France, 1.5 kV SS are generally connected to a three-phase high voltage (HV) grid with rated voltages equal to 63 kV or 90 kV at a frequency of 50 Hz in order to comply with the French Electricity Transmission Network (Réseau de transport d'Electricité or RTE) and a rated voltage of 20 kV for connection to the Electricity of France (Electricité De France or EDF) [148].

In a DC railway network, two superconducting cable topologies can be considered: (i) unipolar and (ii) bipolar. Fig. 27(a) illustrates the unipolar DC HTS cable configuration, consisting of a single positive conductor that carries current from the AC/DC rectifier output (1.5 kV) to the injection point on the traction line. In contrast, Fig. 27(b) presents the bipolar topology, which uses two concentric conductors (positive and negative) to provide separate paths for forward and return currents. Among these, the unipolar configuration is generally regarded to be more reliable and easier to integrate as it does not require modifications to the existing network. However, bipolar topology offers a performance advantage by reducing voltage drops along the traction

line through direct current return to the substation, avoiding resistive losses of the running rails [148].

The DC HTS cables consist of multiple concentric layers of superconducting tape wound around a LN₂ pipe, which serves as the former and provides cooling to maintain the operating temperature in the range of 68–78 K, ensuring that nitrogen remains in a liquid state [145]. Fig. 28 (a1) shows a 3D representation of a typical bipolar coaxial cable, where the positive pole is positioned above the corrugated metallic former. For the present design, the cable comprises 27 superconducting tapes, enabling it to carry a nominal current of 4.4 kA. The tapes in each winding layer are staggered by approximately 0.3–0.6 mm, depending on the twist pitch defined by the winding machine. Fig. 28 (a2) depicts a sketch of a commercial 2G HTS tape used for the winding, and the inset (a3) gives a photograph of such a tape. The tape has a width, $w_T = 4$ mm, and a total thickness, $th_T \sim 200$ µm, comprising (i) a superconducting layer $th_{HTS} = 1$ µm, (ii) a substrate for mechanical reinforcement (Hastelloy) with $th_{Has} = 100$ µm, and (iii) a copper stabilizer ($th_{Cu} = 100$ µm) that provides both electrical and mechanical protection [232]. An electrical insulation layer separates the positive and negative poles, ensuring safe operation of the coaxial design [148].

Kraft paper immersed in liquid nitrogen is used as insulation material due to its high dielectric strength of ~50 kV/mm [233]. This paper ensures a fixed separation between the two poles of the cable. However, in the event of a fault, nitrogen bubbling can occur, which significantly reduces the dielectric performance of the insulation [234]. Under such conditions, the dielectric strength may drop from about 50 kV/mm to as low as 2–3 kV/mm at cryogenic temperatures [235,236]. For the present application – namely, a 1.5 kV DC railway system – a paper
<figure><img src="image_27.png" /><figcaption>**Fig. 24.** SuperRail installation at Vouillé substation in the distance of Paris Montparnasse station [230]. (a) View of the entire location (Substation Ceinture-Ouest), including the railway tracks and the substation itself. (b) illustrates the very risky situation to build new rights of way with one century old constructions and the presence of a lot of other networks (water, natural gas, telecommunications). The track is a remain from the former Paris petite ceinture railway from 1867. (c) Existing rights of way saturated with only few conduits left (100 mm in diameter). A copper cable with a diameter of 400 mm² corresponds to ~500 A current. (d) Sketch of the location, side view. The superconducting feeder cable is indicated in blue. (e) Sketch of the location, top view. These images are part of the SuperRail project [230].</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>**Fig. 25.** Schematic view of the cooling system in the normal operation with a Reverse Turbo-Brayton (RTB) unit, which is being developed as part of the SuperRail project [230].</figcaption></figure>

insulation thickness of 6 mm was chosen to provide an additional safety margin. The negative pole is positioned on top of this insulation layer and serves as the return path for the current back to the power source. The entire cable assembly is housed inside a flexible cryostat, which is sub-cooled by pressurized liquid nitrogen. The operating range for temperature and pressure is 68 K < T<sub>LN2</sub> < 78 K and 3 bar < p < 15 bar, respectively [237]. Fig. 28(b) presents the nitrogen phase diagram and highlights the selected operating window to guarantee the liquid phase.

The cooling system is designed to ensure both the initial cool-down and the long-term maintenance of the superconducting cable at its target cryogenic temperature. Fig. 28(c) shows the schematic of

the main components of this system. At its core, the system includes a storage tank containing boiling liquid nitrogen at approximately 77 K. The size of this tank and, consequently, its operating autonomy, depends primarily on the total thermal losses of the installation, which originate from the superconducting cable itself, the cable terminations, and the cooling infrastructure. A sub-cooler is connected to the storage tank via an expansion valve. This unit reduces the temperature of the liquid nitrogen by lowering the bath pressure. The sub-cooled liquid nitrogen is maintained at approximately 64 K, which is only a few hundred millikelvins above the nitrogen solidification point of 63.14 K. Operating at this reduced temperature minimizes bubble formation
<figure><img src="image_29.png" /><figcaption>Fig. 26. Setup of the DC feeding cable for tests at the SNCF Railway Test Agency (SNCF-AEF). (a): View on the experimental installation. The type test loop is 35 m long, with 2 terminations, 1 joint and 2 cable pieces. (b): Shows the LN₂ circulation on the skid, the controlled cool-down system, and the inlet and out terminations T1 and T2. (c): Schematic sketch of the cooling system, including the heat exchanger and the liquid nitrogen pump immersed in the pumped LN₂ bath. These images are part of the SuperRail project [230].</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>Fig. 27. Schematic view of the introduction of a DC superconducting cable to the railway network. (a) unipolar cable, (b) bipolar cable. Source: Reproduced with permission from Hajiri et al. [148].</figcaption></figure>

within the cryostat, ensuring stable thermal contact and maintaining a high dielectric strength of the insulation. This approach also increases the safety margin against local boiling during transient load conditions, thereby improving the overall reliability of the superconducting cable system.

With the above operating conditions in mind, the DC HTS cable was modelled by incorporating the fundamental limits of the superconducting state. A superconductor transitions from its superconducting state to the normal-resistive state when any of the three critical parameters are exceeded: the critical temperature, $T_c$, the critical current density, $J_c$, and the upper critical magnetic field, $H_{c2}$. For a well-cooled cable operating near its nominal cryogenic temperature, two parameters are of primary concern for safe and reliable operation, $J_c$ and the magnetic field. Both parameters must be evaluated to ensure that the

cable remains in the superconducting state under all expected load conditions. $J_c$ is inferred from the critical current, $I_c$, of the cable as

$$J_c = A_{sc} I_c, \quad (1)$$

where $A_{sc}$ denotes the total cross-sectional area of the superconducting material. The critical current $I_c$ itself depends on the magnetic field (which varies with the transport current) and the temperature (assumed here to be nearly constant). The upper critical magnetic field $H_{c2}$ is primarily determined by the superconducting material composition and is often expressed in terms of the critical magnetic flux density, $B_{c2} = \mu_0 H_{c2}$ [157]. These critical parameters were incorporated into the electromagnetic model to ensure an appropriate operating margin [148]. While protection and stability are essential for system-level integration, a well-designed HTS cable can tolerate short-term excursions into the
<figure><img src="image_31.png" /><figcaption>Fig. 28. (a1) 3D representation of a typical bipolar coaxial cable. (a2) View of the different layers of a typical 2G HTS tape. (a3) Photograph of a piece of HTS tape. (c) Nitrogen phase diagram and the chosen operating area of the HTS cable (in grey) to ensure the liquid phase. (d) Simplified diagram of the different components of the cooling system.</figcaption></figure>

Source: Reproduced with permission from Hajiri et al. [148].

resistive state, e.g., for a few AC power cycles or limited durations in DC operation without compromising the system stability or long-term cable reliability.

For the numerical implementation, the electromagnetic response of the HTS cable was modelled using COMSOL Multiphysics [148]. The superconducting state was represented using a nonlinear resistivity law, $\rho(J, B, T)$ of the form

$$ \rho(J, B, T) = \rho_n \left( \frac{J}{J_c(B, T)} \right)^{n-1}, \quad (2) $$

where $\rho_n$ denotes the normal-state resistivity, $J$ the local current density and $n$ is the power-law exponent characterizing the $E-J$ relationship of the HTS material, which is about 20-40 for coated conductors. This formulation allows representing the transition from the superconducting state ($J < J_c$) to the resistive state ($J \ge J_c$) smoothly, enabling stable transient simulations. The critical current density $J_c(B, T)$ was determined from experimentally derived scaling laws like

$$ J_c(B, T) = J_{c0} \left( 1 + \frac{|B|}{B_0} \right)^{-\alpha}, \quad (3) $$

where $J_{c0}(T)$ denotes the zero-field critical current density, $B_0$ is a scaling field, and $\alpha$ is a material-dependent parameter. This nonlinear constitutive law allows the numerical model to smoothly transition from the superconducting to resistive state, capturing the dynamic behaviour of the cable during normal operation, as well as during fault or transient conditions.

These critical quantities are subsequently incorporated into the superconductor electromagnetic model to determine the appropriate current margin required for safe and reliable cable operation. While protection and stability are essential considerations in any power system, an appropriately designed HTS cable can tolerate temporary excursions into the normal-resistive state. In practical terms, this capability allows the cable to operate safely in the resistive regime for several AC power cycles or for a limited duration under DC conditions, without compromising the overall stability and integrity of the power

system [148]. According to Hajiri et al. [148] to estimate this operating margin accurately, it is essential to use a model that combines both the intrinsic physics of the superconductor and the geometric characteristics of the cable. The cable geometry plays a critical role in shaping the magnetic field distribution and influencing local temperature variations, both of which directly affect its impedance. The finite element (FE) method is particularly well suited for this task, as it enables a detailed representation of the cable’s physical structure and electromagnetic behaviour, thus providing a robust framework for assessing safe operating limits.

The coaxial configuration of the cable exhibits symmetry in the transverse plane, which enables a simplified numerical approach. Instead of modelling the full 360° geometry, only a single sector of the cable is considered, significantly reducing computational complexity. This symmetry-based simplification reduces the mesh from 165,714 domain elements and 6172 boundary elements (full model) to just 5849 domain elements and 422 boundary elements (sector model). Each superconducting tape is discretized into 100 elements, corresponding to a mesh size of approximately 40 µm, providing sufficient resolution for accurate calculation of magnetic field and current density [148]. Modelling of the Electrical Traction Network

An essential step forward to achieve a better understanding how the traction network is influenced by the installation of a superconducting feeder cable, a full modelling of the electrical traction network was carried out. In Ref. [148], the 1.5 kV DC traction network is modelled using lumped parameters, with the system response evolving dynamically according to the position and power demand of trains operating between substations. A finite element (FE) electromagnetic model of the HTS cable was coupled with a lumped-parameter circuit model of the traction network, implemented using COMSOL Multiphysics Electrical Circuit interface. The circuit model includes the substations (AC/DC conversion and feeders), overhead lines as distributed elements, return paths through running rails, and the trains modelled as dynamic, time-dependent current sinks based on their position and traction power requirements. The HTS cable was represented as a non-linear
<figure><img src="image_32.png" /><figcaption>**Fig. 29.** (a) Representation of the forces acting on the train during motion. (b1) Simplified diagram showing data exchange between the two software packages. (b2) Conceptual diagram of the co-simulation coupling between Simulink and the COMSOL FEM model, where *B* is the magnetic flux density, and *J*<sub>HTS</sub> is the current density in the superconducting tape. (c) Equivalent impedance diagrams of the traction line: (c1) with a unipolar superconducting cable (UniHTSC). (c2) with a bipolar superconducting cable (BiHTSC). (c3 and c4) illustrate the current flow directions for two train positions (P1 and P2) relative to the injection point for the unipolar case. For simplicity, these two positions are represented by switches P1 and P2: (c3) corresponds to the train located between *p*<sub>SS1</sub> and *p*<sub>HTS</sub> (P1 closed, P2 open), while (c4) corresponds to the train located between *p*<sub>HTS</sub> and *p*<sub>SS2</sub> (P1 open, P2 closed).
*Source:* Reproduced with permission from Hajiri et al. [148].</figcaption></figure>

impedance element in the circuit, with its resistance updated at each time step using data from the FE model. The FE model incorporates the dependence of the superconducting resistivity on current density, magnetic field distribution, and operating temperature. A co-simulation approach was used to capture interactions between the power network and the superconducting cable under transient conditions. This allowed for the evaluation of voltage drops along the traction line, energy savings within the network, and loss reduction attributed to the HTS cable. The approach enables realistic transient simulations incorporating both train motion and non-linear electrical behaviour of superconductors, providing valuable insights into the potential benefits of HTS technology in railway traction networks.

Fig. 29(a) illustrates the forces acting on the train during motion. The rolling resistance force is denoted as $F_{\text{rol}}$, the aerodynamic drag force as $F_{\text{dyn}}$, and the gradient force, due to track slope or inclination, as $F_{\text{grd}}$. The air density is represented by $r_a$ (kg/m$^3$), $v$ is the train speed (m/s), $S_T$ is the frontal area of the train and $g = 9.81 \text{ m/s}^2$ is the gravitational acceleration. The aerodynamic drag coefficient and the rolling resistance coefficient are given by $C_T$ and $F_{\text{ro}}$, respectively. The total train mass is $m_t$, and $\alpha$ denotes the track inclination angle.

Fig. 29(b) illustrates the coupling layout between the two software packages, implemented using the Simulink LiveLink™ toolbox of COMSOL 5.6. The left diagram shows the exchange of variables between Simulink and COMSOL: Simulink processes global variables such as voltage and current, while COMSOL computes distributed variables such as current density and magnetic field. In this study, the communication time step between the software environments is user-defined and fixed at 0.1 ms. From an electrical perspective, as illustrated on the right of Fig. 29(b), the superconducting cable is represented by an equivalent resistance $R_{\text{eq}}(t)$ and an equivalent inductance $L_{\text{eq}}(t)$. The cable resistance $R_{\text{eq}}$ is derived from the ratio of electrical losses $Q$ to

the square of the supplied current $I_{\text{HTS}}$, while the equivalent inductance $L_{\text{eq}}$ is obtained from the magnetic energy $W_m$ computed in COMSOL.

Fig. 29 (c1, unipolar) and (c2, bipolar) illustrate the traction line circuit as a function of the train position, $p_{\text{Tk}}(t)$. The train position $p_{\text{Tk}}$ varies along the section between substation 1 ($p_{\text{SS1}}$) and substation 2 ($p_{\text{SS2}}$). The impedances of the traction line are recalculated at each time step based on the instantaneous train position. Here, $Z_l$ and $Z_r$ represent the impedances of the overhead line and the return rail, respectively. The overhead line resistance is 28 mΩ/km and the rail resistance is 16 mΩ/km, while their respective inductances are 0.8 and 0.7 μH/km. To account for the movement of the train along the traction line, two electrical circuits are solved as illustrated in Fig. 29 (c3) and (c4): (i) Circuit 1: Train located between SS1 and the superconducting cable injection point $p_{\text{HTS}}$, i.e., $p_{\text{Tk}}(t) \in [p_{\text{SS1}}, p_{\text{HTS}}]$, and (ii) Circuit 2: Train located between $p_{\text{HTS}}$ and SS2, i.e., $p_{\text{Tk}}(t) \in [p_{\text{HTS}}, p_{\text{SS2}}]$ (see Fig. 30).

For the bipolar cable model (BiHTSC) as shown in Fig. 29 (c1), two switches are installed, one upstream and one downstream of the train to represent the two operating scenarios as the train travels from substation SS1 to SS2. In the unipolar configuration (UniHTSC, see Fig. 29 (c2)), only a single switch is required, positioned on the overhead traction line, since the HTS cable carries the forward current from the substation to the injection point while the return current flows through the rails. When the train is located between SS1 and the HTS injection point $p_{\text{HTS}}$ (Fig. 29 (c3)), switch P1 is closed and switch P2 is open. After the train passes the HTS injection point, the switch states are reversed (Fig. 29 (c4)), with P1 open and P2 closed. This ensures proper current routing through the HTS cable for all train positions along the line.

The traction network parameters were kept identical for all simulation cases. The model comprises two identical substations, each with
<figure><img src="image_33.png" /><figcaption>**Fig. 30.** Comparison of the voltage and current of train T1 for three cases: conventional, UniHTSC, and BiHTSC. (a) Pantograph voltage of train T1 for case 1 (conventional: —, UniHTSC: - - - , BiHTSC: - - - - ). (b) Current drawn by train T1 for case 1 (same colour coding as in (a)). On the right side, magnified views of the current and voltage of the train at different time instants are shown. (c) Total energy consumption of the traction line for the three cable technologies in case 1, where both trains follow the same dynamic behaviour.</figcaption></figure>

*Source:* Reproduced with permission from Hajiri et al. [148].

a nominal power rating of 6.6 MW and capable of delivering up to 10 MW of peak power. The no-load voltage of each substation is set to 1750 V. The substations are positioned 10 km apart, with SS1 located at $p_{SS1} = 0$ m and SS2 at $p_{SS2} = 10$ km.

The trains commonly operating on the considered line belong to the Z 20900 series, part of the Z2N family of SNCF. These trains are capable of reaching a maximum speed of 140 km/h and can be supplied with either 1.5 kV DC or 25 kV/50 Hz AC power. For the present modelling, only the DC operation mode was considered, with a rated power of 3.464 MW. The train develops a tractive force of 310 kN and has an unloaded mass of 245 t. It measures 103.50 m in length and has a frontal area of $2.82 \times 4.32 \text{ m}^2$. Although these trains are designed for speeds up to 140 km/h, SNCF typically evaluates their performance using a steady-state speed of 60 km/h, which was also adopted in this study.

In Ref. [148], a refined finite element (FE) model was developed and coupled via co-simulation with a circuit model representing a traction network section comprising two substations connected to the power grid. The FE model incorporated detailed superconducting physics, enabling an in-depth evaluation of two superconducting cable topologies: unipolar and bipolar. The case study considered two trains operating under distinct speed profiles, including scenarios with simultaneous departures and staggered departures from the two substations. The traction network model was parameterized using data provided by SNCF within the framework of the SuperRail project, with the objective of assessing the potential of superconducting cables to improve traction line electrical efficiency and reduce voltage drops in the Parisian railway network. The results of Ref. [148] shown in Fig. 30 demonstrated that the bipolar superconducting cable delivers the most favourable performance, achieving a 5%–10% reduction in overall energy consumption and an 8%–13% reduction in voltage drop compared to conventional solutions. Although the use of superconducting cables requires a dedicated cryogenic cooling system, the bipolar topology was shown to offer lower operational costs due to its substantial energy savings. The proposed modelling methodology is therefore well-suited for techno-economic assessments, enabling railway operators and infrastructure planners to accurately estimate the return on investment (ROI) of adopting superconducting cable technology.

Another key issue identified during the SuperRail project concerns the design and performance of cable terminations required for superconducting feeder cable installations. Unlike conventional copper-based

feeders, superconducting cables operate at cryogenic temperatures and require specialized terminations to interface between the cryogenic cable environment and the ambient conditions of the traction power network. These terminations must simultaneously: (i) maintain electrical insulation suitable for 1.5 kV DC operation, (ii) ensure thermal isolation to minimize heat ingress into the cryogenic system, (iii) provide a reliable mechanical interface to standard railway infrastructure, and (iv) incorporate diagnostic and protection systems to safely handle fault scenarios. Preliminary tests within the project revealed that the termination design has a significant impact on the overall efficiency of the system, given that the feeder cables are relatively short. It was found that approximately 30% of the total losses occur within the cryostat itself, while as much as 70% of the losses are attributed to terminations. Fig. 31 (a–c) illustrate possible termination configurations, the implemented solution for the Vouillé substation, and the measured loss distribution.

Future work should therefore focus on optimizing termination designs, evaluating the required number of terminations, and improving their integration within railway substations to ensure that the performance benefits of superconducting feeder cables can be fully realized without introducing additional operational risks.

A first optimization, carried out in [170], reduced losses in the terminations of a railway feeder cable by 37%, compared to the standard approach. This was achieved by sizing the termination according to the average daily current rather than the rated current, and cooling it by half its length with liquid nitrogen.

### 5.3.2. Superconducting SMES and FCL for railways

In addition to superconducting cables, superconducting magnetic energy storage (SMES) and fault current limiters (FCL) represent promising technologies for enhancing railway traction power systems [32,178–182]. The current status of these systems was reviewed in Refs. [20,21], and applications in railway systems were reviewed in Refs. [30,31]. Thus, we mention here only the advantages of these developments when being connected to a railway network.

SMES units can store energy in the magnetic field of a superconducting coil and release it nearly instantaneously when required [20]. In railway applications, SMES can stabilize voltage by providing fast-response support during transient load changes caused by train acceleration or regenerative braking, recover braking energy locally, reducing
<figure><img src="image_34.png" /><figcaption>**Fig. 31.** (a) Possible solutions for cable terminations, including a conventional design, the SuperRail demonstrator (TRL 6–7), and the laboratory demonstrator (TRL 4) which has been investigated within the SuperRail project. (b) Locations of the terminations at the Vouillé substation and at the platform side. (c) Schematic representation of the losses associated with the terminations.</figcaption></figure>

overall energy consumption, and improve substation load management, enabling higher train density or integration of renewable power sources [30,31]. However, SMES systems require cryogenic cooling and have high upfront costs, which currently limit their deployment to pilot projects and feasibility studies.

Superconducting FCLs exploit the transition from the superconducting to the resistive state to limit fault currents without introducing permanent impedance during normal operation [21]. For DC and AC railway power networks, superconducting FCLs provide [35,183–192] several advantages like rapid current limitation, protection of substations and traction lines during short circuits, reduced stress on switchgear and overhead line components, as well as increased network reliability and operational safety. Prototype superconducting FCLs for railway networks have been demonstrated in Europe and Asia, showing significant reduction of peak fault current while allowing quick restoration of normal service.

Thus, one can state here that these two technologies are well developed and practically ready for being installed within railway networks. These technologies, when combined with superconducting cables, offer a pathway towards fully superconducting traction feeder systems, delivering increased efficiency, higher power density, and improved fault tolerance.

## 5.4. Summary of the developments for conventional electrified railways

To summarize this part, the bibliometric analysis presented in Section 2 identified 78 articles addressing the use of superconductivity in conventional, electrified railways, corresponding to 67% of all publications in our literature survey. The majority of these contributions originate from European and Japanese research groups, reflecting the regions with the highest density of conventional, electrified railway networks.

Most articles focus on improving the supporting grid to ensure stable and efficient energy delivery (SMES, FCL and feeder cables). Notably, recent projects in Japan and France successfully implemented superconducting feeder cables for DC-electrified railway lines, clearly demonstrating the feasibility of such infrastructure upgrades.

In addition, several studies explored the integration of superconducting components onboard electric vehicles, including motors, transformers, and braking systems. While these technologies have been developed and tested at the prototype level, their large-scale introduction into locomotives and trainsets has not yet been realized.

Overall, the literature indicates that infrastructure-related applications of superconductivity in railways have already achieved the stage of practical implementation, whereas the onboard applications remain at the experimental stage and face significant barriers to deployment.

## 6. Conclusions and outlook

The integration of superconductivity into railway systems represents a transformative step towards highly efficient, sustainable, and high-capacity transport infrastructure. The research efforts are divided into two main categories:

* (i) the development of superconducting maglev trains, and

* (ii) the use of superconductivity for conventional, electrified railway systems.

Table 2 provides a comparison of the research directions for integrating superconductivity into railway systems (superconducting maglev, as well as the network of conventional, electrified railways), highlighting their purposes, technologies, infrastructure needs, benefits, challenges, and feasibility.

Recent progress in maglev technology is closely linked to advances in (RE)BCO HTS tapes, which enable operation at liquid nitrogen
<table><caption>**Table 2**
Comparison of research directions for integrating superconductivity into railway systems (maglev and the network of conventional, electrified railways), highlighting their purposes, technologies, infrastructure needs, benefits, challenges, and feasibility.</caption><thead><tr><th>Aspect</th><th>Superconducting maglev systems</th><th>Superconducting power & energy supply and storage</th></tr></thead><tbody><tr><td>Purpose</td><td>Achieve frictionless, high-speed and propulsion</td><td>Improve efficiency of railway power supply and storage</td></tr><tr><td>Key Technology</td><td>HTS magnets for levitation and propulsion</td><td>Superconducting cables for power transmission, Superconducting Magnetic Energy Storage (SMEs)</td></tr><tr><td>Cooling Requirements</td><td>Cryogenic cooling (LN<sub>2</sub> or LHe) for superconducting magnets</td><td>Cryogenic cooling for superconducting cables and SMES coils</td></tr><tr><td>Infrastructure Needs</td><td>Specially designed guideways, dedicated lines</td><td>Integration with existing railway substations and power grids</td></tr><tr><td>Main Benefits</td><td>Speeds >600 km/h, near zero friction, low maintenance</td><td>Almost zero electrical losses, peak load management, efficient regenerative braking</td></tr><tr><td>Challenges</td><td>High cost of tracks and superconducting magnets, reliability under vibration, safety</td><td>Cooling costs, mechanical stability, high SMES cost, retrofitting complexity</td></tr><tr><td>Examples</td><td>JR-maglev, China's HTS maglev prototypes</td><td>Pilot superconducting cable projects, grid-level SMES demonstrations</td></tr><tr><td>Feasibility Outlook</td><td>Limited to dedicated new lines for high-speed intercity transport</td><td>Incremental adoption in existing networks, especially urban and regional lines</td></tr></tbody></table>

temperatures. A key milestone is the JR-Maglev, currently scheduled for commercial operation in 2034, which holds the world train speed record of 603 km/h. The transition from the MLX-01 prototype to the Series L0 highlights substantial improvements in superconducting technology, with potential spillover benefits for conventional electrified railways.

New developments extend maglev applications beyond demonstrators and people movers to high-speed flux-pinning systems, a capability unique to superconducting materials. In parallel, modern superconductors enable alternative designs beyond the original JR-maglev, which relied on low-temperature superconducting (LTS) coils and liquid helium (LHe) cooling.

The Hyperloop concept provides additional impetus for the use of superconductivity for levitation and propulsion, envisioning maglev-based transport at near-aircraft speeds (~1000 km/h), which will further accelerating superconducting technology development in the near future.

The second category, conventional electrified railways, can be divided into two subcategories: Superconducting applications within the vehicles themselves and those in the supporting railway grid for energy delivery.

The first subcategory is still in its early stages but stands to benefit significantly from JR-maglev developments, particularly when the Chuo Line begins commercial operation. A key focus is the use of superconducting wireless power transfer (WPT) to supply energy directly to vehicles [171–177]. Once a superconducting system is required to be installed onboard, other superconducting components are likely to follow, leveraging the shared cryogenic infrastructure. Early research has already resulted in prototypes of highly efficient superconducting transformers [203–210] and motors [201,202]. More recently, superconducting eddy-current brakes have been developed, offering advantages such as greater clearance from the rails, which simplifies installation and maintenance [193]. The integration of on-board superconducting fault current limiters (FCLs) can significantly enhance system availability by rapidly suppressing fault currents, minimizing damage, and reducing recovery times. On-board superconducting magnetic energy storage (SMES) systems can provide a compact, high-power solution for bridging non-electrified sections or catenary faults, enabling “last-mile” operations without auxiliary diesel engines [238]. By eliminating the need for fossil fuel-based backup, SMES reduces

local emissions, lowers noise pollution in urban or enclosed environments, and supports the broader transition to fully electrified, low-carbon rail transport. Together, these advances suggest a clear pathway towards fully superconducting drivetrains, enabling highly efficient, compact, and low-maintenance railway vehicles of the future.

Here, we further mention application possibilities, which are not included in the list of papers investigated here: Superconductivity may also play a significant role in auxiliary railway systems, particularly in compact and efficient cooling solutions such as HVAC (Heating, Ventilation, and Air Conditioning). This can be achieved through magnetic refrigeration [239,240], which utilizes HTS coils to generate high magnetic fields. These strong fields enable compact, high-performance refrigeration units capable of providing efficient cooling for passenger compartments as well as for onboard power electronics. Since railway vehicles operate under strict space and energy constraints, the use of compact and energy-efficient cooling systems is highly advantageous. In addition, superconducting levitation technologies can be applied to rotating equipment, where superconducting bearings reduce friction and mechanical wear. This leads to lower maintenance requirements, increased equipment lifespan, and improved overall reliability of railway systems.

Overall, superconductivity in the railway network provides the following key benefits:

(i) Weight reduction and increased efficiency: High power density reduces train mass and improves acceleration profiles.

(ii) Energy saving: On-board energy storage reduces peak power demand from substations.

(iii) Maintenance reduction: Superconducting bearings and components decrease mechanical wear.

In contrast, the second subcategory has already reached the status of a real application in the railway network. Superconducting feeder cables, fault current limiters (FCL), and magnetic energy storage systems (SMES) offer distinct advantages that align with the growing demands for electrified railway networks.

(i) Superconducting feeder cables.

The deployment of HTS cables in traction networks can significantly reduce transmission losses and mitigate voltage drops in long feeder lines. Their ability to carry very high currents in
compact, lightweight designs enables upgrades to existing networks without extensive civil engineering work. Bipolar HTS DC feeders in particular demonstrate notable efficiency improvements (5%-10% energy savings) and enhanced voltage stability. Future developments are likely to focus on improving cryogenic reliability, modularity of cable terminations, and integration with existing maintenance strategies.

### (ii) Superconducting fault current limiters (FCL)

FCL technology can protect railway power systems against high short-circuit currents, especially in dense urban networks where prospective fault currents exceed conventional circuit breaker capabilities. Superconducting FCLs operate inherently without additional control, providing rapid and self-resetting current limitation. Continued advancements in cost-effective and reliable cryogenic systems are essential to make FCL integration standard in high-capacity substations. Superconducting fault current limiters and DC link inductors onboard can improve safety and stability of traction power systems, especially in hybrid or multi-system trains.

### (iii) Superconducting magnetic energy storage (SMES)

SMES offers instantaneous power support during transient events, improving voltage stability during train acceleration or regenerative braking. It can also facilitate grid balancing, especially when railway power systems are integrated with renewable energy sources. Onboard SMES modules can capture regenerative braking energy and return it during acceleration, reducing energy drawn from the overhead catenary and improving overall energy efficiency. However, the relatively high capital cost and cryogenic cooling requirements of SMES remain a challenge for large-scale adoption.

The now existing installations of superconducting feeder cables in France and Japan have demonstrated their ability to significantly reduce transmission losses, enhance power supply stability, and handle high current densities, confirming their suitability for modern railway power systems. However, certain technical challenges, such as optimizing the cable terminations, still require further research. Moreover, local space constraints play a crucial role in driving the adoption of superconductivity within railway networks, as compact cable designs can offer significant advantages in densely built environments.

The potential of superconductivity in railways is closely tied to advances in cryogenics, particularly towards maintenance-free, long-life cooling solutions. Economic competitiveness will improve as material costs decline and as modular superconducting components become commercially mature. The convergence of superconductivity with digital railway management, renewable integration, and carbon-neutral operation targets positions superconducting technology as a strong candidate for next-generation railway power systems.

Finally, we provide in the following a personal opinion of the authors. The progress in the development of superconducting tapes (coated conductors based on (RE)BCO) and bulk (RE)BCO samples has already enabled several successful applications of superconductivity in the railway sector. A prominent example is the French Super-Rail project, which demonstrated the installation of a superconducting feeder cable at a substation near Montparnasse station in Paris. In parallel, the Railway Technical Research Institute (RTRI, Japan) has achieved significant progress with superconducting feeder cables for subways and DC-electrified railway lines. These results indicate that the supporting networks of railways, particularly DC-operated systems, can clearly benefit from the use of superconductivity. In addition, electromagnetic modelling plays a crucial role, providing insights that range from the design of single components like trapped field magnets to the optimization of the entire electrical traction network with installed superconducting feeder cables.

By contrast, superconductivity has not yet been introduced into conventional electric railway vehicles. In particular, the deployment of

superconducting transformers in AC-electrified railway systems, which were since long considered a promising application, remains unrealized. Superconducting maglev technologies, however, have profited strongly from these material advances. JR Central's EDS maglev has recently been equipped with its first vehicle carrying high-T$_{c}$ magnets, while projects in China have integrated bulk superconductors for levitation purposes. Moreover, the HTS pinning maglev, which was previously limited to demonstrators and low-speed people-movers, has taken a decisive step towards high-speed operation with the launch of a full-scale prototype train at Southwest Jiaotong University, China. An important aspect here is that this development also places new demands on material research, since bulk superconductors were not originally optimized to minimize AC losses under dynamic operating conditions.

These advances are further stimulated by research inspired by the Hyperloop concept, which continues to attract strong academic and industrial attention. It is therefore reasonable to expect that ongoing progress in superconducting technologies for maglev and low-pressure transport systems will also accelerate their eventual adoption in conventional railways. In this way, superconductivity could become a key enabler in meeting the demands of the Sustainable Development Goals (SDGs) by providing more efficient, sustainable, and high-capacity transport solutions.

## List of Abbreviations

<table><tr><td>2G</td><td>Second generation</td></tr><tr><td>AC</td><td>Alternating Current</td></tr><tr><td>Bi-2223</td><td>(Pb,Bi)<sub>2</sub>Sr<sub>2</sub>Ca<sub>2</sub>Cu<sub>3</sub>O<sub>10+δ</sub></td></tr><tr><td>BiHTSC</td><td>Bipolar HTS feeder cable</td></tr><tr><td>ASC</td><td>Applied Superconductivity Conference</td></tr><tr><td>CRRC</td><td>Changchun Railway Vehicles Co., Ltd.</td></tr><tr><td>DB</td><td>Deutsche Bahn AG</td></tr><tr><td>DC</td><td>Direct current</td></tr><tr><td>EDS</td><td>Electrodynamic Suspension</td></tr><tr><td>EDF</td><td>Electricité de France</td></tr><tr><td>EMC</td><td>Electromagnetic Compensation</td></tr><tr><td>EMS</td><td>Electromagnetic Suspension</td></tr><tr><td>EUCAS</td><td>European Conference on Applied Superconductivity</td></tr><tr><td>FCL</td><td>Fault-current Limiter</td></tr><tr><td>FEM</td><td>Finite Element Modeling</td></tr><tr><td>HTS</td><td>High-temperature Superconductors</td></tr><tr><td>HVAC</td><td>Heating, Ventilation, and Air Conditioning</td></tr><tr><td>IBS</td><td>Iron-based superconductors</td></tr><tr><td>ICE</td><td>Intercity Express</td></tr><tr><td>IEEE</td><td>Institute of Electrical and Electronics Engineers</td></tr><tr><td>ISM</td><td>Induction/Synchronous Motor</td></tr><tr><td>JR</td><td>Japan Railways</td></tr><tr><td>LECB</td><td>Linear Eddy Current Brake</td></tr><tr><td>LGV</td><td>Ligne à grande vitesse (French high-speed line)</td></tr><tr><td>LHe</td><td>Liquid Helium</td></tr><tr><td>LN2</td><td>Liquid Nitrogen</td></tr><tr><td>LTS</td><td>Low-temperature superconductors (requiring LHe-cooling)</td></tr><tr><td>MOCVD</td><td>Metal-Organic Chemical Vapor Deposition</td></tr><tr><td>OA</td><td>Open Access</td></tr><tr><td>PM</td><td>Permanent Magnet</td></tr><tr><td>PMG</td><td>Permanent Magnet Guideway</td></tr><tr><td>(RE)BCO</td><td>(RE)Ba<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> with RE = rare Earth elements</td></tr><tr><td>SDGs</td><td>Sustainable Development Goals</td></tr><tr><td>SMES</td><td>Superconducting Magnetic Energy Storage</td></tr><tr><td>SNCF</td><td>Société Nationale de Chemins de Fer (French Railways)</td></tr><tr><td>St.E.S.</td><td>Studiengesellschaft für elektrische Schnellbahnen (Research Association for High-speed electric railways)</td></tr></table>
<table><tr><td>RR</td><td>Railroad</td></tr><tr><td>RTB</td><td>Reverse Turbo Brayton</td></tr><tr><td>RTE</td><td>Réseau de transport d'Electricité (French electricity network)</td></tr><tr><td>RTRI</td><td>Railway-Technical Research Institute, Kokobunji, Tokyo, Japan</td></tr><tr><td>SS</td><td>Sub-station</td></tr><tr><td>TGV</td><td>Train à grande vitesse (French high-speed trains)</td></tr><tr><td>UFRJ</td><td>Federal University of Rio de Janeiro</td></tr><tr><td>UM</td><td>Universal Mechanism</td></tr><tr><td>UN</td><td>United Nations</td></tr><tr><td>UniHTSC</td><td>Unipolar HTS feeder cable</td></tr><tr><td>WET</td><td>Wireless energy transfer</td></tr><tr><td>WPT</td><td>Wireless Power Transfer</td></tr><tr><td>WoS</td><td>Web of Science</td></tr></table>

### CRediT authorship contribution statement

**Michael Rudolf Kobischka:** Writing – review & editing, Writing – original draft, Visualization, Investigation, Formal analysis. **Kévin Berger:** Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Conceptualization. **Hervé Caron:** Writing – review & editing, Resources, Funding acquisition.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

The authors would like to thank the SuperRail project, which was funded by the French government via the Banque Publique d'Investissement (BPI) as part of the France 2030 initiative.

### Data availability

Data will be made available on request.

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