The EMC E1 was an early passenger-train diesel locomotive developing 1,800 hp, with an A1A-A1A wheel arrangement, and manufactured by Electro-Motive Corporation of La Grange, Illinois. They were built during 1937 and 1938 for the Atchison, Topeka and Santa Fe Railway for a new generation of diesel-powered streamlined trains. 8 cab-equipped lead A units and three cabless booster B units were built. The initial three locomotives were AB pairs built to haul the Santa Fe's Super Chief diesel streamliners, while the others were built as single A units to haul shorter trains. The locomotives were diesel-electrics with two 900 hp (670 kW) Winton 201-A engines each, with each engine driving its own generator to power the traction motors. The E1 was the second model in a long line of passenger diesels of similar design known as EMD E-units. All Winton 201A-engined Santa Fe passenger units, including the E1s, were extensively rebuilt into the 80-class E8M engines in 1952–53. These were similar to production E8 models, but derated to 2,000 hp so as not to burn out the early traction (axle) motors.

A B unit, in railroad terminology, is a locomotive unit which does not have a control cab or crew compartment, and must therefore be operated in tandem with another coupled locomotive with a cab. The terms booster unit and cabless are also used. The concept is largely confined to North America. Elsewhere, locomotives without driving cabs are rare.