B unit
Some B units cannot be moved without a controlling unit attached, but most have some simple controls inside, and often a side window at that control station. For example, B unit versions of the EMD FT with conventional couplers had a fifth porthole-style window added on the right side only for the control station. Other models used existing windows. These controls enable a hostler to move the B unit locomotive by itself in a yard or shops. B units without controls are generally semi-permanently coupled to controlling units. Sometimes, there is a terminology distinction between the types: a 'booster' is a B unit with hostler controls, and a 'slave' is a B unit without hostler controls.