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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 be a number of self-powered cars or a set of locomotives.

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


South Side Elevated Railroad car #1, one of the cars that Frank Sprague converted to MU operation in Chicago
The multiple unit traction control system was pioneered 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 world wide.

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.

Locomotive applications

Sprague's MU system was adopted for use by diesel-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.

Most modern diesel locomotives are now delivered equipped for MU operation, allowing a consist (set) of locomotives to be operated from one cab. However, not all MU connections are standardized between manufacturers, thus limiting the types of locomotives that can be used together. In North America there is a high level of standardization using the Association of American Railroads system . In the United Kingdommarker several different incompatible MU systems are in use (and some locomotive classes were never fitted for MU working), but more modern diesel locomotives used on British railways use the standard AAR system.

Modern locomotive MU systems can be easily spotted due to the large MU Cables to the right and left of the coupler. The connections typically consist of several air hoses for controlling the air brake system, and an electrical cable for the control of the traction equipment. The largest hose, located next to the coupler is the main air brake line or "train line". Additional hoses link the air compressors on the locomotives and control the brakes on the locomotives independently of the rest of the train. There are sometimes additional hoses that control the application of sand to the rails.

Locomotives set up to use slugs have extra connections for transmitting electricity from the diesel electric generator on the "mother" unit to the traction motors on the slug.

Passenger train applications

Modern electric multiple unit and diesel multiple unit vehicles often utilise a specialised coupler that provides both mechanical, electrical and pneumatic connections between vehicles. These couplers permit trains to be connected and disconnected automatically without the need for human intervention on the ground.

There are a few designs of fully automatic couplers in use worldwide, including the Scharfenberg coupler, various knuckle hybrids (such as the Tightlock, used in the UK), the Wedgelock coupling, Dellner couplings (similar to Scharfenberg couplers in appearance), and the BSI coupling.

Multiple control technology is also required for push-pull trains operating with a standard locomotive at one end only. Control signals are either received from the cab as normal, or from a cab car at the other end that is connected to the locomotive by cables through the intermediate cars.


  1. Railway Technical Web Pages:Modern Railway Terminology
  2. US Loco MU Control

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