B unit
In the early 1960s, Union Pacific started asking for a 3-unit "locomotive set" to not only replace their turbine locomotives, but to bring a new generation of diesel locomotives to their rails, and then put them into use. The EMD (Electro Motive Division of General Motors) DD35 was a cabless booster (B unit) locomotive that consisted of two GP35 locomotives prime movers (engines) and electrical systems, mounted on a common frame and covered with a single cowling, riding on a pair of 4-axle Flexicoil trucks, so each prime mover was powering a single set of 4 axles instead of two sets of 2 axles, as in the GP35. Being a "B-unit", the DD35 lacked any sort of cab, and could only be run on the main line when connected with another "A-unit" to control it. The 4-axle truck was considered too harsh on the trackwork to be in the lead, and thus EMD's response to UP's request was a 4-unit (instead of the requested 3-unit set) locomotive set, consisting of a pair of 2,500 hp GP35s sandwiching a pair of DD35s, to make one 15,000 hp locomotive set (basically six GP35's in four units)