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

MU system was adopted for use by diesel-electric locomotives in the 1920s, however these early connections were entirely pneumatic. Todays 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, thereby eliminating the need for a train crew in each engine (something steam locomotives necessitated). However, not all MU connections are standardized between manufacturers, 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 [1].

Modern locomotive MU systems can be easily spotted as 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.