## 1 Introduction  
Easy and affordable access to the fast transportation of goods and passengers is a key driver of social and economic growth, yet it also contributes significantly to climate change. Addressing this dilemma, the hyperloop concept emerges as a promising solution, balancing both opportunities and challenges. The vision of connecting cities with the speed of airplanes, the throughput of subways, the flexibility of buses and the ecological footprint of trains is inspiring engineers, researchers and entrepreneurs around the world. The term hyperloop has become popular since being introduced in a white paper by [PERSON] in 2013 [1]. Hyperloop refers to a ground-based, public transportation system, operating at the speed of a commercial aircraft. For this, so-called pods travel in a near-vacuum tube to reduce air resistance. Emerging from a student competition, the TUM Hyperloop research project was founded at the Technical University of Munich in 2019. One major goal was to develop a 24 m full-scale demonstrator. The system depicted in Figure 1 was successfully built, certified and presented in 2023 [2]. This study addresses the complex challenge of providing a transparent hyperloop concept design as a baseline for further improvement in the community. To achieve the ambitious goals of the TUM hyperloop project, a comprehensive technical concept analysis was imperative. This analysis ensures the selection of the most effective concept from a set of assumptions, directly aligning with the central research question of this work. The process is particularly crucial from an infrastructure perspective, where limited opportunities for iteration necessitate a forward-thinking design approach. Moreover, the increasing demand from regulatory authorities for the standardization of the hyperloop system highlights the need for a transparent process and the meticulous evaluation of each design decision. Given yearlong development cycles, maintaining the coherence and traceability of evolving assumptions and consequent decisions becomes challenging. The design decision tree (DDT) formalism [3; 4], not to be confused with the decision tree classifier methodology [5; 6], was adopted for this purpose. The DDT is an approach commonly used for evaluation and documentation, mainly for complex software architectures [3; 4], but also in other areas like strategic business development [7] or psychology [8]. Based on the formalism, a framework was developed to suit the development of complex hardware designs. The derived structured approach was used to visualize, document and explain the design decisions made during the concept phase and helped to keep the team aligned during the design and construction phases. The DDT formalism supports the incremental design and documentation process but is no replacement for systems engineering approaches. Several hyperloop companies have published ideas regarding the technical concept of suspension and propulsion. A detailed elaboration is available from the company Hardt Hyperloop [9], featuring a concept where the pod hangs from the ceiling of the tube and is propelled by a short stator reluctance motor. Additionally to this, maglev concepts, like the German Transrapid [10], the Japanese Maglev [11], or the German Transport System Bogl (TSB) [12], are valuable references for already tested concepts of magnetic suspension and propulsion. Also, in the 1990s, research on the Grumman Superconducting Electromagnetic Suspension design was published [13; 14], showcasing and testing on a test stand an interesting concept for superconducting propulsion and suspension. The focus of the research community is on specific system challenges, such as aerodynamic drag [15; 16], with only a few studies [17] addressing the technical concept of hyperloop. This work seeks to answer the following specific research questions:  
1. How can a framework be developed to incrementally visualize, document and refine the design processes of complex hardware systems, such as the hyperloop?