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—with only a control signal transmitted to each unit. This contrasts with arrangements where electric motors in different units are connected directly to the power supply switched by a single control mechanism, thus requiring the full traction power to be transmitted through the train.

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

Origins

Multiple unit train control was first used in electric multiple units in the 1890s.

The Liverpool Overhead Railway
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.

Frank J. Sprague
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.

Locomotive applications
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, a train would require two train crews as the four A units could not be multiple-unit-controlled, except as two groups of two.
 
Terms used in North America are A unit and B unit where the B or "slave" unit does not have a control cab; slug where the B unit has traction motors powered by the "mother" unit via extra connections; and Cow-calf for switcher units. A Control Car Remote Control Locomotive has remote control but not traction equipment.

Most modern diesel locomotives are now delivered equipped for MU operation, allowing a consist (set) of locomotives to be operated from one cab. Not all MU connections are standardized between manufacturers, thus limiting the types of locomotives that can be used together.  However, in North America there is a high level of standardization between all railroads and manufacturers using the Association of American Railroads (AAR) system which allows any modern locomotive in North America to be connected to any other modern North American locomotive. In the United Kingdom 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 Association of American Railroads 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.

With distributed power, long trains, e.g. ore trains on mining lines, may have locomotives at each end and at intermediate locations in the train to reduce the maximum drawbar load. The locomotives are often radio-controlled from the lead locomotive by the Locotrol system. Remote control locomotives, e.g. "switchers" in hump yards, may be controlled by a stationary operator. These types of remote control systems often use the AAR MU standard which allows any locomotive using the AAR MU standard to be easily "MU'ed" to a control receiver and thus capable of becoming remote-controlled.

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 used in 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.

In the United States Amtrak often operates one to three diesel locomotives on routes outside the Northeast corridor with only one operator.