## 1 Introduction
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Country**} & \multicolumn{2}{c}{**Percentage of**} & \multicolumn{2}{c}{**Percentage 2 of Railway**} \\  & **Non-Electrified Lines** & \multicolumn{2}{c}{**in Freight Traffic**} \\ \cline{2-5}  & **2013** & **2022** & **2013** & **2022** \\ \hline Austria & 30.32 & 27.98 & 32.1 & 30.2 \\ Denmark & 76.25 & 67.24 & 10.9 & 9.6 \\ France & 48.09 & 39.74 & 10.8 & 10.6 \\ Germany & 47.37 & 45.16 & 19.3 & 19.8 \\ Poland & 38.59 & 37.28 & 27.6 & 23.2 \\ Slovakia & 56.32 & 56.31 & 36.5 & 30.4 \\ Spain & 48.09 & 36.16 & 5.3 & 4.4 \\ Sweden & 25.03 & 25.00 & 33.7 & 28.7 \\ UE 27 1 & 46.19 & 43.19 & 21.0 & 16.0 \\ \hline \hline \end{tabular} \({}^{\dagger}\) European Union without UK (2013). 2 Only land transport methods.  
\end{table} 
Table 1: Percentage of non-electrified lines and percentage of railway in freight traffic (Europe, a perspective from 2013 to 2022). Fuel cells have emerged as a popular alternative to electrification, though they still present notable disadvantages, such as lower efficiency compared with batteries and the need for dedicated refueling infrastructure. Nevertheless, fuel cell technology offers distinct advantages, enabling the operation of intercity and regional services (with ranges of approximately 800 km) at low-traffic service routes and with shorter refueling times. The functions of OESD have also been analyzed in [8] and can be grouped into five main categories. It is worth noting that these functions closely resemble the conventional operation of Distributed Energy Resources (DER) in Public Power Systems (PPS):  
* Power assist (peak clipping): OESD can supply power during acceleration phases, partially offsetting the demand on the primary energy source. This functionality enables a reduction in the rated power needed for hybrid units. * Regenerative braking: Energy recovered during braking can be used to power auxiliary loads or other units. However, it is often challenging (or even impossible) to recover all this energy in real time, making energy storage essential. A similar situation occurs with prosumers in renewable energy systems when excess generation exceeds demand, and injecting energy back into the grid is either technically unfeasible or economically inefficient. * Operation on non-electrified tracks: OESD facilitates operations on these segments, such as \"last-mile\" applications near cities or industrial tracks. This reduces the need for shunting locomotives, lowering costs and minimizing environmental impact. * Assistance in emergencies: OESD can provide power in the event of catenary (Overhead Line Equipment, OLE) failures caused by meteorological factors or mechanical faults, such as issues with the pantograph-catenary contact. This functionality is analogous to a \"black start\" service in PPS. Unfortunately, some railway operators have overlooked the potential of this capability. * Load leveling service: By balancing energy demands, OESD can enhance the efficiency of the railway power system (RPS). This approach is closely aligned with PPS strategies for optimizing energy generation and reducing peak energy prices. OESD, and specifically supercapacitors (Electric Double-Layer Capacitors, EDLC), offer significant advantages in the operation of locomotives and Diesel Multiple Units (DMUs), particularly when hybridized with batteries. These advantages include:
* Improved fuel efficiency during idle operations: EDLCs enable multiple restarts of diesel units without consuming fuel during idle periods. This is particularly beneficial for commuter locomotives and DMU services. For instance, in low-traffic lines, passenger locomotives and DMUs often spend 40-60 min idling on platforms while waiting to begin a new service, as observed in the selected scenario. * Support for peak current demands: EDLCs can deliver fast peak currents during the initial acceleration phase, which is a significant challenge for batteries and often necessitates oversizing them.