Push–pull train
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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 in the UK, cab car in the US) at the other end.
Historically push-pull trains with steam power provided the driver with basic controls at the cab end along with a bell or other signalling code system to communicate with the fireman located in the engine itself in order to pass commands to adjust controls not available in the cab. At low speeds some push-pull trains are run entirely from the engine with the guard operating bell codes and brakes from the leading cab when the locomotive is pushing the train. The Corris Railway operates in this fashion today. Many mountain railways also operate on similar principles in order to keep the locomotive lower down than the carriage so that there is no opportunity for a carriage to run away from a train down the gradient, and also so that if the locomotive ever did run away it would not take the carriage with it. Modern train control systems use sophisticated electronics to allow full remote control of locomotives. Nevertheless push-pull operation still requires considerable design care to ensure that control system failure does not endanger passengers and also to ensure that in the event of a derailment the pushing locomotive does not push a derailed train into an obstacle worsening the accident.
When operating push-pull the train can be driven either from the locomotive or the alternate cab. 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 alternate cab. This configuration ensures that the locomotive never needs to be uncoupled from the train, and ensures fast turnaround times at a railway station terminus.
In certain situations the locomotive is placed in the middle of the train rather than at one end but driven from cabs at the train ends. The GWR did this when multiple carriages were linked up in an autocoach train as the mechanical linkages used to control the train were not capable of reliable operation through a train unlike modern electrical and pneumatic control systems. When the locomotive is placed mid-train both directions are considered 'push'.
Alternatively a push-pull train, especially a long one, may have a locomotive on both ends so that there is always one locomotive pushing and one locomotive pulling. In this case caution must be used to make sure that the two locomotives do not put too much stress on the cars from uneven locomotives. This two-locomotive formation is used by the Intercity 125 and its Australian equivalent the XPT. It is usual to arrange things so that auxiliary power is supplied by the trailing locomotive so that the locomotive at the front does more pulling than the locomotive at the rear does pushing. Having an independent locomotive as opposed to a power car at each end is also known in the railway world as a top and tail.