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Train Operations Ⅲ
Push-Pull Operation
Push-pull operation was actually only an adaption of the multiple unit principle but applied to a locomotive powered train.Assuming a regular level of traffic and an even interval service was required，trains could be formed with a locomotive at one end and a driving cab on the coach at the other end.If you could find a way of doing it cheaply by changing existing coaches，it could represent a big step forward.
The idea has now been adopted world-wide in two forms.One，as stated above，uses a locomotive at one end and a coach equipped with a driver’s cab at the other end.The number of vehicles between them may be varied seasonally if required but the formation is not usually varied on a train by train basis.In the UK，the coach at the back has become designated a Driving Van Trailer（DVT）.It is used to carry luggage and passengers are not permitted to ride in it at speeds over 160km/h.
The second push-pull form uses two locomotives，one at each end of the train.This was applied to the Channel Tunnel“Le Shuttle”trains and has also appeared elsewhere，noticeably in Taiwan.The two locomotives are necessary in these cases to provide enough power.