A B unit is a type of locomotive that lacks controls or is intended to be used only in conjunction with a fully equipped A unit. These were typically diesel locomotives, and were more popular in the United States than other parts of the world. The largest 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. B units were also less flexible for this same reason, as they could not be used alone without substantial retrofitting. A handful of B units have undergone this type of modification throughout the years, and some A units have been subjected to a reverse conversion by having their controls stripped out.
Most B units were designed so that they could not be moved without an attached control locomotive. Some others included a limited control set so that they could be moved independently around a stock yard. Units without any controls at all were often referred to as slaves, while those with limited controls could be called boosters. Booster units were often referred to as having hostler controls, as a hostler is a railroad employee that is typically authorized to move trains around a stock yard but not the actual railway.
One of the main concerns that often led to the purchasing of B units was cost. Since a B unit lacks controls, crew facilities, and other expensive additions, they are typically less expensive for a railroad company to acquire. Many railroads did not initially consider each diesel locomotive to be a distinct unit, and instead ordered them in precise configurations to replace older steam locomotives. This direct replacement made modularity and flexibility lesser concerns than they would be later on.
Under certain circumstances a B unit could be converted into an A unit, and the opposite also sometimes occurred. One reason for conversion may be if a railroad that does not use B units acquires the rolling stock of another railroad. 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 units are sometimes converted into B units if rebuilding the controls would be too costly. These conversions typically involve stripping out remaining controls, blacking out windows, and removing facilities like toilets. Since a B unit does not typically need to support a crew, any HVAC equipment or other systems devoted to comfort may also be removed.