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Train Operations Ⅳ
Multiple Unit Operation
Locomotive operation of intensive services was fast phased out when electric traction，using“multiple unit”operation，was introduced late last century for US urban railway lines.Within ten years the idea had spread to Europe.The facility for the electric traction system to be spread out along the train，compared with cramming it all together into a bulky locomotive，allowed a number of small power units to be distributed underneath the floors of several vehicles in the train.At the same time they were all controlled by the driver in the leading car through wires running the length of the train.Thus was born the electric multiple unit or EMU.In later years，DMUs（diesel multiple units）were developed using the same principles.
A modern passenger multiple unit train now consists of a number of inter-dependent vehicles which cannot operate unless all the vehicles are of the right type and are coupled in the correct position in the train.Power and auxiliary equipment is generally distributed under more than one vehicle and is all controlled from the driving position.Vehicles in multiple units are usually referred to as“cars”and are known as“motor cars”if powered and“trailer cars”if not.
Multiple unit trains are formed into“units”or“sets”of two or more cars.They are often semi-permanently coupled together，only being uncoupled inside a workshop for heavy maintenance.Units can operate singly—providing driver’s cabs are provided at both ends—or coupled to form longer trains.Some operations need two（or more）multiple units to be coupled together to provide sufficient capacity for a particular service.This also allows trains to be lengthened or shortened while in service by adding or cutting units.
Some multiple unit trains are designed so that a unit has a full driver’s cab at one end only.At least two units，coupled back to back，are needed to make up a train for service.In the US，a development of this type of formation，known as“married pairs”，has been popular since the 1960s.Two cars，coupled together and electrically dependent on each other，form a unit and a number of these are coupled to form trains of four，six，eight etc.cars.Similar formations have since appeared elsewhere，e.g.London Underground’s Central Line.
Multiple unit trains are mainly used for high density suburban operations where traffic levels are easily predicted and form constant patterns which allows fixed train formations.Recently long distance traffics have shown the same tendency and many railways are now adopting the multiple unit formation for these routes—e.g.the French TGV，the UK HST，the Japanese Shinkansen.
Other advantages of the EMU are that it doesn’t have to carry its own fuel，it takes it from overhead wires or an additional（third）rail and it is quick and simple to reverse at terminals.All you have to do was provide drivers controls at both ends of the train，connect them to the train wires and give the driver a key to switch in the controls at the end he wants to use.Locomotive changing is immediately eliminated，terminal space is released and trains can be turned round more quickly.All the driver has to do now is change ends.
Another form of multiple unit operation was adopted in the early 1960s when a new concept appeared called push-pull.