The User to User Signalling Supplementary Service is widely used in the operation of GSM for Railways (GSM-R). The applications 'Presentation of Functional Numbers' [2], [3] and 'Confirmation of High Priority Calls' [4], [5] have been specified, implemented and tested in the framework of national GSM-R schemes. In defining layouts for the new features DSD Alarm Notification and Alerting Dispatcher care has been taken to ensure that existing implementations are not compromised or invalidated when laying out a framework for flexible further extension. For any such further extension, therefore, it is mandatory to define the use of UUIE in these various applications to avoid interoperability issues in the future. This TS defines the contents of the User to User Information Element when used in the GSM-R environment. This includes the basic EIRENE features Functional Addressing, Location Dependant Addressing, Confirmation of High Priority Calls and Presentation of Functional Numbers. In addition this document defines layouts for further features: Enhanced Presentation of Functional Numbers, Enhanced Location Dependent Addressing, Driver Safety Device alarm, Plain Text Messages, Presentation of the Functional Number of the initiator of a Railway Emergency Call and Alerting Dispatcher. Finally, this TS describes the requirements to be followed by network operators to ensure compatibility and interoperability if they wish to define specific fields for national and/or network use.