An A unit is a railway locomotive used as the manned master unit in multiple locomotive train configurations. When trains are configured to be driven by multiple locomotive units, only one unit has a driver; all control inputs are remotely relayed to the others. These control relays generally take the form of a cable link between the locomotives which allows for a direct interface between the A unit's driver cab controls and the B unit's remote control stations. Although older steam locomotives are also often used in tandem, the term A unit typically only applies to diesel/electric or electric-only locomotives. In countries such as the U.S., the locomotives which make up the unmanned B or slave units are custom built without any of the driver cab and amenity features of the A units.
Particularly where rail routes pass through hilly or mountainous terrain, extended trains often require more than one locomotive to haul them efficiently. These multiple locomotive trains may consist of anything from two to four units and sometimes more in exceptional cases. On routes where multiple locomotive configurations are common, railway operators often make use of an A unit locomotive as a master unit with one or more B units supplying traction only. A typical A unit diesel or electric locomotive includes a range of driver control and amenity features such as a control cab, sleeping facilities, a galley, and toilets.
Many American rail operators have their B unit locomotives constructed with no cab, none of the driver amenities, and only rudimentary controls for moving the unit in the rail yard. B unit control in extended trains is exercised by the driver of the A unit via a cable interface between the units. This allows the driver of the A unit to synchronize power outputs, braking, and any other functions of the B locomotives from his unit. A units are typically placed at the head of the locomotive group as that allows for maximum visibility for the drivers. There are situations however, where multiple locomotive groups are led by a B unit.
A unit locomotives may be converted to B types should the need arise. B units may be converted to A units but the more common conversion is A to B due to cost issues. This type of conversion is often considered where A unit locomotives are damaged to the extent where cab rebuilds are not financially viable.