Radio Electronic Token Block

 is a system of railway signalling used in the United Kingdom. It is a development of the physical token system for controlling traffic on single lines. On arrival at a “token exchange point”, the driver reports his/her position to the signaller by radio and requests the “token” for the next section of line ahead. If the signaller is in a position to do so, he/she will issue the electronic token applicable to the section ahead. Simultaneously, the driver must operate a button on an apparatus in the cab to receive the token. The token is then transmitted to the train by radio. The Solid State Interlocking controlling the system prevents the issue of any token permitting conflicting movements. In the same way as with the traditional system, when a physical token with the name of the section engraved on it would be carried in the cab, the electronic token is received and displayed by name on the train equipment. This token is the authority to occupy the single line, and it cannot be removed from the train until the driver him/herself releases it. After receiving the token, the driver is then given verbal permission to pass the “Stop Board” and enter that section; the stop board is used instead of signals and therefore needs no electrical supply. RETB is now regarded by the rail industry as a redundant technology and is being gradually replaced with the new European in-cab signalling system, ERTMS.