  GSM-R is predicted to be obsoleted by 2030, and a suitable successor is
needed. Defined by the International Union of Railways (UIC), the Future
Railway Mobile Communication System (FRMCS) contains many future use cases with
strict requirements. These use cases should ensure regular communication not
only in network coverage but also uncovered scenarios. There is still a lack of
standards on off-network communication in FRMCS, so this article focuses on
off-network communication and intends to provide reference and direction for
standardization. We first provide a comprehensive summary and analysis of
off-network use cases in FRMCS. Then we give an overview of existing
technologies (GSM-R, TETRA, DMR, LTE-V2X, and NR-V2X) that may support
off-network communication. In addition, we simulate and evaluate the
performance of existing technologies. Simulation results show that it is
possible to satisfy the off-network communication requirements in FRMCS with
enhancements based on LTE-V2X or NR-V2X. Finally, we give some future research
directions to provide insights for industry and academia.
