RETB

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. The fixed distant board on the approach has a single permanent AWS inductor which gives a warning in the cab regardless of the signal box instruction and has to be cancelled when passed. Points at the entrance to a crossing loop are spring-loaded for the correct track for facing movements, and are pushed across by the wheels for trailing movements; they too require no power or interlocking, other than for points heating purposes. In the facing direction, a 'points indicator' is provided to indicate to the driver that the points are correctly set. The points indicator is in the form of a yellow light, lit only while the points are electrically detected in the required position. The whole line can be operated by just one or two signallers and needs very little infrastructure other than the track itself, making it a very cost-effective method.

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