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Push-pull is a mode of operation for locomotive-hauled trains. A push-pull train has a locomotive at one end of the train, and an alternative driver's cab ( DBSO or DVT) at the other end.The train can be driven either from the locomotive or the DBSO/DVT. If the train is heading in the direction in which the locomotive end of the train is facing, this is considered 'pulling'. If the train is heading in the opposite direction, this is considered 'pushing', and the driver is located in the DBSO/DVT. This configuration ensures that the locomotive never needs to be uncoupled from the train, and ensures fast turnaround times at a railway station terminus.
The method of communication between the DBSO/DVT and the locomotive is known as " Time-division multiplex" or TDM. Both the locomotive and the DBSO/DVT are fitted with TDM equipment, which is connected using "RCH" jumper cables which run the length of the train. These cables were originally fitted to control the train lighting, but are now also used to transmit public address messages as well as being used for the TDM.
Whenever the driver operates the controls in the DBSO/DVT, the TDM sends a message down the train, which is decoded by the other TDM and then operates the controls in the locomotive. For safety reasons, the air braking system is operated directly from whichever cab the driver is driving from. If the TDM fails and cannot be reconfigured, the train may still operate, but if the DBSO/DVT is leading, it will be necessary to uncouple the locomotive and attach it to the front of the train.
Alternatively a push-pull train, especially a long one, may have a locomotive on both ends. This is the formation used by the Intercity 125.
See also: Rail terminology
