B-Units

In railroad jargon, a B unit refers to a locomotive unit, typically a diesel locomotive, that lacks a control cab or crew compartment. As a result, it requires operation in tandem with another locomotive unit that has a cab (an A unit). The terms "booster unit" and "cabless" are also used to describe B units. It's important to note that this concept is primarily found in North America, as locomotives without driving cabs are uncommon elsewhere.
Controls for B units can vary. Some B units have basic controls inside and often feature a side window at the control station. For instance, B unit versions of the EMD FT locomotive, with conventional couplers, had a fifth window on the right side specifically for the control station. Other models utilized existing windows for this purpose. These controls allow hostlers to move the B unit locomotive on its own in rail yards or shops. B units without controls are typically semi-permanently coupled to controlling units. Sometimes, there is a terminology distinction between the types: a "booster" is a B unit with hostler controls, while a "slave" is a B unit without hostler controls.
There are several reasons why railroads ordered B units. One significant factor was cost savings, as B units were slightly cheaper than their A unit counterparts. B units lack certain features found in A units, such as windshields, crew seats, radios, heating, air conditioning, and even toilets, which were commonly located in the short hood of an A unit. Additionally, early on, railroads purchased multiple-unit diesel locomotives as direct replacements for steam locomotives. This meant that railroads couldn't take full advantage of the flexibility offered by interchangeable units that could be assembled into various power configurations. As B units lacked controls, unions couldn't insist on staffing each unit. Lastly, B units provided a streamlined appearance for passenger service, enhancing the train's aesthetic.
The era of B units was most common during the 1930s, 1940s, 1950s, and 1960s, especially during the cab unit locomotive era. However, as hood unit road switchers became more prevalent, many railroads concluded that the cost savings of B units didn't outweigh the loss of operational flexibility. Consequently, few B units have been constructed in the last four decades. Some Western railroads, like the Southern Pacific, Union Pacific, Burlington Northern, and the Santa Fe, continued ordering B units longer than most. In fact, Santa Fe ordered the GP60B model in 1991, marking the final B units built for road service in North America as of 2005.
In some cases, B units are converted from existing A units. This conversion process typically involves removing the cab or blanking out its windows and eliminating non-essential equipment like speedometers, event recorders, horns, headlights, toilets, and cab heaters. These conversions were often undertaken when A units were damaged in collisions, and rebuilding the cab was not cost-effective. Some rare instances saw B units converted to include a cab, as was the case with the "Crandall Cabs" on the Chicago & North Western Railway in the 1970s. The Santa Fe also rebuilt some of its GP7Bs to GP9us with cabs.
Occasionally, a B unit may run at the front of a train, although this is typically avoided due to limited visibility from the locomotive cab. Nonetheless, operational requirements or locomotive orientation might necessitate having the B unit at the front.