This papers reports on activities in the European project MOTIONAL that aims at the development of a digital twin environment which facilities the modularity, interoperability and composability of complex digital twin assemblies of railway systems. The approach that refers to the Functional Mock-up Interface is justified by a discussion of the comparable activities in industry and in the automotive field compared to particularities in the railway system. The work was initiated by the selection and analysis of nine use cases. An introductory digital twin example illustrates the current implementation status and related aspects, while an outlook presents the integration into the Federated Rail Data Space as the business case and as a vision of the activity.