## 4 Initial Taxonomy of Issues  
The next step was to build a preliminary taxonomy of research topics. As explained in Section 3, all papers were read and grouped under the different clusters. Each paper was also flagged for the respective research issues. Table 8 aggregates the findings from the 161 documents. For the utility clusters, an overview of the emerging issues is reported, while for the physical and generic clusters, the research issues are reported in detail, aggregating the identified issues from Section 3. It should be noted that the obtained taxonomy is not unique, and further readings could identify additional elements. A variety of researched topics emerges from Table 8.  
The Hyperloop as a system cluster (A) includes a lot of research on different operational aspects, in particular communications. In fact, this aspect appears to be challenging at very high speeds in tunnel structures. Some other aspects related to the geometric design and the linear infrastructure development are also covered in this cluster in an analytical manner. The Hyperloop substructure cluster (B) includes a great deal of research from the fields of structural and bridge engineering. The major difference is the dynamic loads \begin{table}
\begin{tabular}{l l l l} \hline \hline  & \multicolumn{2}{c}{**Research Clusters**} & \multicolumn{2}{c}{**Researched Issues**} \\ \hline  & 1 & Energy & Energy consumption (may include aerodynamics, but focuses on heat dissipation) \\ \cline{2-4}  & 2 & Safety & Safety process, evacuation, pod tightness, breaking \\ \cline{2-4} Utility cluster & 3 & Communications & Pod-to-pod and pod-to-ground communication \\ \cline{2-4} overview & 4 & Aerodynamics & Aerodynamic phenomena \\ \cline{2-4}  & 5 & Operations & Hyperloop operations and research not covered in utility clusters 1–4 \\ \cline{2-4}  & A & System & Optical wireless communication, pod-to-ground communication, communication signal propagation, system simulation, functional safety, process safety, safety cases, energy storage systems, lateral drift, energy consumption, network architecture, scheduling, short range communication, power requirements, impact on the electricity grid, short-range communication, scheduling, electric and software system, photovoltaic panels, handover algorithm, geometric design, linear infrastructure planning \\ \cline{2-4} Physical clusters & B & Substructure & Structural design, bridge dynamics, geotechnical, earthquake, resonant dynamic effects, vibration instability, bridge deck-per interaction, bridge dynamics, dynamic amplification factors, dynamic load amplification, floating Hyperloop tunnel \\ \cline{2-4}  & C & Tube & Airtightness, anomaly detection \\ \cline{2-4}  & D & Tube-pod interface & Levitation friction, aerodynamic drag, blockage ratio, vacuum effects, piston effect mitigation, heat generation, tube/pod combined design, energy loss, aerodynamic noise, levitation force, kinetic energy, pressure recycle ducts, aerodynamic breaking \\ \cline{2-4}  & E & Pod & Motor, propulsion, semi-active suspension, electrodynamic suspension, levitation, guidance, design, sizing, battery, tightness, Linear Induction Motor, high-temperature superconducting (HTS) magnet, batteries, wheel design, additive manufacturing, inverter for powertrain, Cold Gas Propulsion \\ \cline{2-4} Generic clusters & i & Discussion & Technical feasibility, financing, policy recommendations, new mobility paradigms, knowledge management, technology overview, education, competitions, general feasibility \\ \cline{2-4}  & ii & Network & Network feasibility, financial efficiency, network simulations, network operations, scenario analysis, synchromodality, supply chain, regional impact \\ \hline \hline \end{tabular}
\end{table} 
Table 8: A taxonomy of overarching research clusters and research issues on Hyperloop arising from the scientific literature analysis. imposed by the Hyperloop pods, which influence the design of substructure and need to be accounted for. Some research deficiencies were identified.