Future Railway Mobile Communication System (FRMCS) will be the 5G SA MCX standard for railway operational communications, conforming to European regulation. It will also fulfil the needs of rail organisations worldwide. UIC (International Union of Railways) is leading the design, standardisation, and is deeply involved for the introduction plan of this telecommunication system, in close cooperation with railways, authorities and suppliers. FRMCS is the successor of GSM-R which is a component of European Railway Traffic Management System (ERTMS), representing around ~130,000 km of coverage of tracks in Europe. GSM-R is however announced to become obsolete soon due to its 2G technology. Therefore, FRMCS, being specified and implemented as a standard, combining 5G transport with Mission Critical (MC) service layer features, is considered as a major trigger for the wide-ranging digitalization of the rail sector, satisfying the increasing demand of data while keeping the high quality of service for critical railways applications, in an interoperability context. This paper aims to present some insights of the FRMCS introduction plan and provide an overview of the outcomes of 5GRAIL, an EU Horizon 2020 project coordinated by UIC, as part of the FRMCS readiness initiative. For more information, visit: https://uic.org/rail-system/telecoms-signalling/article/frmcs , 5GRAIL - 5G for future RAILway mobile communication system.