An A unit is a manned master unit in multiple locomotive train configurations, controlling the B units through a cable link. B units lack driver amenities and are controlled by A unit conductors. A units can be converted to B types if necessary.
An A unit is a railway locomotive used as a manned master unit in multiple locomotive train configurations. When trains are set up to be driven by multiple locomotive units, only one unit has a driver; All control inputs are remotely transmitted to the others. These control relays generally take the form of a cable link between the locomotives which allows direct interface between the A unit’s driver’s cab controls and the B unit’s remote control stations. Although older steam locomotives Older ones are also often used in tandem, the term A unit generally only applied to diesel/electric or electric-only locomotives. In countries like the US, the locomotives that make up the B units or unmanned slaves are custom built without any of the cab and driver features of the A units.
Particularly where rail routes pass through hilly or hilly terrain, stretched trains often require more than one locomotive to haul them efficiently. These multiple locomotive trains can consist of two to four units, and sometimes more in exceptional cases. On routes where multiple locomotive configurations are common, rail operators often use one A-unit locomotive as the master unit with one or more B-units supplying traction only. A typical A-unit diesel or electric locomotive includes a variety of control features and driver amenities, such as a control cab, sleeping facilities, a galley, and toilets.
Many American railroad operators have their B-unit locomotives built with no cab, none of the driver’s amenities, and only rudimentary controls for moving the unit around the rail yard. Control of unit B in extended trains is exercised by unit A’s conductor through a cable interface between the units. This allows the driver of unit A to synchronize the power outputs, braking and any other function of the locomotives B from his unit. Units are usually positioned at the head of the locomotive group, as that allows maximum visibility for the drivers. However, there are situations where multiple groups of locomotives are run by one B unit.
A-unit locomotives can be converted to B-types if the need arises. B units can be converted to A units, but the most common conversion is A to B due to cost concerns. This type of conversion is often considered where the A-unit locomotives are damaged to the extent that cab rebuilds are not financially viable.
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