## 2 Materials and Methods  
The methodology presented in this section focuses on capturing research findings, aiming at the identification of trends, and consequently, building a taxonomy of issues. The Scopus database, which has scrupulous indexing rules, was used as a source. For the analysis, the following steps were taken:  
* A search using specific keywords (\"Hyperloop\" or \"tube transport\" or \"vactrain\") was carried out, in the abstract, title, or keywords. Results were limited to those published after 2013 (when the modern concept of Hyperloop was introduced), and documents from health sciences were excluded due to the lexical ambiguity of \"Hyperloop transport\" term. The exact query used was: TITLE-ABS-KEY (\"Hyperloop\" OR \"tube transport\" or \"vactrain\") AND PUBYEAR > 2013 and not SUBJAREA (MEDI OR NURS OR VETE OR DENT OR HEAL). This search performed in June 2021 resulted in 229 documents. * An additional manual filtering of the documents one-by-one, on the basis of their title or abstract limited, resulted in 161 documents. The aim of this filtering was to eliminate those documents that were not relevant to the field due to lexical ambiguity and those that simply outlined Hyperloop-related aspects. This left 96 articles, 57 conference papers, three reviews, three notes, one letter and one book chapter. Figure 1 shows the distribution of the documents over the considered time period. Figure 1 shows an overview of the results, which are destined to increase in 2021. After this step, an analysis of all abstracts (and in case of doubt, of the full paper) took place, and the research was quantitatively assessed, focusing on several clusters. Inspired by the decomposition approach from [14], this was done by means of a system approach, breaking the Hyperloop system into five physical parts (Figure 2). These parts cover the entire hyperloop system, and outline interacting subsystems. Figure 1: Evolution of Hyperloop academic research. The five physical clusters are:  
* Hyperloop as a system: this includes research that encompasses the entire system and that cannot be considered under other disaggregated levels. Examples may include efficiency and energy studies of the system in operation. * Substructure (including foundations and bridge work): focuses mostly on structural engineering design for the supporting structure. * Tube: considers aspects related to the tube structure. * Tube pod interface: focuses on research on the interface between the tube and the pod. Examples may include aerodynamic phenomena as a consequence of the pressure variation. * Pod: focuses on aspects related to the pod (e.g., levitation, suspension, powertrain, electronics)  
In addition, five horizontal (operational) clusters (energy, operations, communications, aerodynamics, safety) were considered. It should be noted that this decomposition (into five physical and five horizontal clusters) while meaningful, is not the only one possible. In fact, in a design process it is impossible to decompose a system uniquely [17]. Nevertheless, this provides a rather generic and complete higher-level decomposition, which can be further broken down into lower hierarchies. For example, the \" pod\" cluster can be further decomposed into sub-clusters, covering the powertrain, the levitation and suspension blocks, etc. Likewise, the horizontal clusters can be further elaborated to cover additional operations. In this sense, the decomposition is scalable and provides the starting point for adding more elaborated layers of detail. These clusters, although developed independently for this study, also encompass and are aligned with the priority work areas identified by the CEN/CENELEC TC on Hyperloop standardization, which include pressures of operation, door sealing, vehicle-tube interface, communication protocols and emergency evacuation [13]. Sections 3.1-3.5 present the results for the five physical clusters. In the analyses, each paper is also linked to one of the five horizonal clusters.