Introduction
Originally derived from lift operation over a hundred years ago, multiple unit (MU) control has become the most common form of train control in use around the world today. This page describes how it started and its development in the century to date.
See also Electric Traction, Electric Traction Drives, Electric Traction Glossary and Electronic Power.
Contents
Origins
Electric locomotives were originally designed so that the motors were controlled directly by the driver. The traction power circuits passed through a large controller mounted in the driving cab. A handle was rotated by the driver as necessary to change the switches in the circuit to increase or reduce power as required. This arrangement meant that the driver had to remain close to the motors if long and heavy, power-carrying cables were to be avoided.
While this arrangement worked well enough, the desire to get rapid turnrounds on city streetcar railways led to the adoption of remote control. The idea was that, if the motors could be remotely controlled, a set of driver's controls could be placed at each end of the train. It would not be necessary to have a locomotive added at the rear of an arriving train to allow it to make the return journey. A cab would be installed at each end of the train and the driver just had to change ends to change direction. Once this idea was established, it was realised that the motors could be placed anywhere along the train, with as many or as few as required to provide the performance desired. With this development, more but smaller motors were scattered along the train instead of building a few large motors in a locomotive. This is how the concept of motor cars and trailer cars evolved. Trailer cars are just passenger carrying vehicles but motor cars are passenger carrying vehicles which have motors and their associated control equipment.
Multiple unit trains, as these trains became known, were equipped with control cables called train lines, which connected the driver's controls with the motor controls and power switches on each motor car. The opening and closing of the power switches was achieved by electro-magnetic relays, using principles originally designed for lifts. While the idea was being established on passenger trains, it was also adopted on locomotives. It quickly became the standard method of control.
Full article: http://www.railway-technical.com/muops.shtml
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