Editing Multiple-unit train control (section)

==Locomotive applications==
[[File:F40PH Illustrated Hoses and Cables.png|thumb|[[EMD F40PH]] Locomotive with image edited to illustrate the location and functions of the various receptacles and hoses featured on many AAR Standard North American locomotives. The communication jumpers (outermost yellow) are exclusive to passenger locomotives and are omitted from freight locomotives.]]
Sprague's MU system was adopted for use by [[diesel–electric locomotive]]s and [[electric locomotive]]s 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 systems; some were compatible with one another, but others were not. For example, when first delivered, many [[EMD F-unit|F units]] lacked MU cables on their noses, allowing only for MUing through the rear of the locomotive. That meant that if a train needed four locomotives and there were four [[A unit]]s and no [[B unit]]s, 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 "booster" unit does not have a control cab; [[slug (railroad)|slug]] where the B unit has traction motors powered by the "mother" unit via extra connections; and [[cow–calf]] for [[switcher locomotive]] units. A control car remote control locomotive has remote control but not traction equipment.

Most modern [[diesel locomotive]]s 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.<ref>{{ cite web | url=http://railway-technical.com/trains/rolling-stock-index-l/diesel-locomotives/us-locomotive-mu-control.html | title=US Locomotive MU Control {{!}} The Railway Technical Website }}</ref> In the [[United Kingdom]] several incompatible [[Multiple working|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 [[Railway coupling|coupler]].  The connections typically consist of several air hoses for controlling the [[air brake (rail)|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 locomotive]]s, 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.