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B units are locomotives without controls that are used in conjunction with fully equipped A units. They are less flexible but cheaper to purchase. Some B units have undergone modifications to become A units, and damaged A units may be converted to B units.
The AB unit is a type of locomotive that lacks controls or is designed to be used only in conjunction with a fully equipped A unit. These were typically diesel locomotives, and they were more popular in the United States than in other parts of the world. The greatest benefit of a B unit was typically the lower costs associated with the lack of controls, crew facilities, and heating, ventilation, and air conditioning (HVAC) equipment. The B units were also less flexible for this very reason, as they couldn’t be used on their own without substantial modification. A handful of B units have undergone this type of modification over the years, and some A units have undergone a reverse conversion by stripping off their controls.
Most of the B units were designed so that they could not move without a control locomotive attached. Some others included a limited control set so they could move independently around a pen. Units without any control were often referred to as slaves, while those with limited control could be called reinforcements. Booster units were often referred to as having hostler controls, as a hostler is a railway employee who is usually authorized to move trains around a stock depot, but not the actual railway.
One of the main concerns that often led to the purchase of B units was cost. Since a B unit lacks controls, crew facilities, and other expensive additions, they are generally less expensive for a rail company to purchase. Many railroads did not initially consider each diesel locomotive to be a distinct unit, instead ordering them in precise configurations to replace older steam locomotives. This drop-in replacement made modularity and flexibility less of a concern than it would later be.
Under certain circumstances, a B drive could become an A drive, and sometimes the reverse was true as well. One reason for the conversion may be if a railway that does not use B units purchases the rolling stock from another railway. In cases like this, each B unit would typically have controls, crew facilities, and HVAC equipment installed. Converted locomotives are sometimes referred to as having a Crandall cab.
Damaged A drives are sometimes converted to B drives if rebuilding the controls would be too expensive. These conversions typically involve removing the remaining controls, darkening the windows, and removing facilities such as toilets. Since a B unit does not typically need support for a crew, any HVAC equipment or other dedicated comfort systems may also be removed.
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