## 6 Conclusions
### Outlook  
DTs offer diverse and novel fields of application in logistics systems of German rail transport, whereby their relevance has been confirmed in other industries already (see Section 2.2). Since DTs pave the way to the vision of a digitally networked rail transport operation that enables complex analyses and simulations of interactions and effects of decisions, further research and application is needed (see Sections 5.2.3 and 5.2.4 and Table 2).  
In this context, finding regulatory frameworks and technical standards that enable interoperable operation of DTs in a DT network (DTA) are a major implementation hurdle of DTs (see Figure 2). Due to the sheer complexity and necessary consent of many stakeholders, the exploration within strategic partnerships of sufficient size that have the power to establish standards, is highly recommended (see Section 5.2.3).  
In addition, data are an increasing challenge in the realization of DTs. Current regulatory and contractual frameworks make data sovereignty difficult for system operators. Corresponding parallels were shown in other industries (see Section 5.2.4), leading to a potential conflict in the use of operational data. This area of conflict must therefore be addressed by for instance changing historical contractual structures (see Table 2).  
Regarding the developed GIS (see Figure 7), further research is recommended as well. The GIS can be applied to already existing and operating assets. Hereby, the extension to assets that are still under development (DTP) was identified as promising (e.g., in the inclusion in tenders and developments of ICE trains) should therefore, be pursued (see Section 5.2.1). In addition to the described strategic bundling of DT activities (see Table 2), which enable a standardized and future-oriented GIS application, subsequent research should focus on validating and evolving the developed GIS in a multiple case study design. It can serve to identify correlations between technical and supporting processes as well as to create empirical values in the implementation of DTs. Based on these empirical values, a GIS of individual asset groups, such as ICE trains, can be enabled. In the long term, it thus, contributes to the standardized and targeted evolution of DTs and the operational and environmental improvement of the German rail transport.  
Conceptualization, S.J. and R.G.; methodology, R.G.; validation, R.G.; formal analysis, R.G.; investigation, R.G.; writing--original draft preparation, R.G.; writing--review and editing, R.G.; visualization, R.G.; supervision, S.J.; project administration, R.G. All authors have read and agreed to the published version of the manuscript.  
The corresponding author wishes to acknowledge financial support by Deutsche Bahn AG. Furthermore, both authors acknowledge support by the German Research Foundation and the Open Access Publication Fund of TU Berlin.  
Not applicable.  
The corresponding author would like to thank the Chair of Logistics of the TU Berlin as well as Think Tank Digitalization and Technology and the experts of the Deutsche Bahn AG for the support of this work.  
The authors declare no conflict of interest.