The EMC E2 was an American passenger-train diesel locomotive which as a single unit developed 1,800 horsepower (1,300 kW), from two (2) 900 horsepower (670 kW) prime movers. These locomotives were typically operated as a unit set or ; where the three unit lashup developed 5400 horsepower. This was almost the ideal horsepower required for the tonnage of a 15 - 18 car passenger train, operated over the ruling grades of virtually all of the mileage between major American cities. The units were of the A1A-A1A wheel arrangement, and manufactured by Electro-Motive Corporation (EMC), later Electro-Motive Diesel (EMD) of La Grange, Illinois.

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.