Multiple-unit train control

Multiple-unit train control, sometimes abbreviated to multiple-unit or MU, is a method of simultaneously controlling all the traction equipment in a train from a single location, whether it is a Multiple unit comprising a number of self-powered passenger cars or a set of locomotives.

A set of vehicles under multiple unit control is referred to as a consist in the United States.

Multiple unit train control was first used in Electric Multiple Units in the 1890s.

The Liverpool Overhead Railway opened in 1893 with two car electric multiple units, controllers in cabs at both ends directly controlling the traction current to motors on both cars.

The multiple unit traction control system was developed by Frank Sprague and first applied and tested on the South Side Elevated Railroad (now part of the Chicago 'L') in 1897. In 1895, derived from his company's invention and production of direct current elevator control systems, Frank Sprague invented a multiple unit controller for electric train operation. This accelerated the construction of electric traction railways and trolley systems worldwide. Each car of the train has its own traction motors: by means of motor control relays in each car energized by train-line wires from the front car all of the traction motors in the train are controlled in unison.

Sprague's MU system was adopted for use by diesel-electric locomotives and electric locomotives in the 1920s, however these early control connections were entirely pneumatic. Today's modern MU control utilizes both pneumatic elements for brake control, and electric elements for throttle setting, dynamic braking and fault lights.

In the early days of diesel electric MUing there were numerous different systems, some were compatible with one another, but others were not. For example: when first delivered, many F units lacked MU cables on their noses, allowing only for MUing through the rear of the locomotive. That meant that if a train need four locomotives and there were four A units and no B units that a train would require two train crews as the four A units could not be multiple unit controlled, except as two groups of two. Furthermore, Baldwin Centipedes were infamous for being unable to MU with any other locomotive even other Baldwin diesels, these locomotives could only be MUed with one other locomotive of the same type.

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