Multiple-unit Train Control - Locomotive Applications

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 and while some were compatible with one and other, some 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.

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. 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 (AAR) system. 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 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.

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.