Cab Secure Radio (CSR) was an in-cab analogue radiotelephone system formerly used on parts of the British railway network. Its main function was to provide a secure speech link between the train driver and the signaller which could not be overheard by other train drivers. In areas where CSR was used it had to be the primary method of communication between driver and signaller, always being used in preference to the signal post telephone. CSR was replaced by the GSM-R digital system, forming the initial phase of rollout of ERTMS throughout the UK.