The Future Railway Mobile Communications System (FRMCS) is a project led by the International Railway Union (UIC) to replace GSM-R with a modern digital radio system for railways. Initial user requirements were defined in 2010, with more detailed requirements published in 2016. FRMCS will utilize 3GPP-based 4G/5G communication solutions and is expected to use the extended GSM-R frequency range for critical communications, with the possibility of accessing other frequency ranges for non-critical applications.