A motive power unit driven by a diesel prime mover. In North America, nearly all are diesel-electric, meaning the diesel engine turns an alternator or generator to produce electricity, which then powers traction motors geared directly to the axles. This design provides maximum torque at low speeds and eliminates the need for a mechanical transmission.
We've a pair of heavy diesel locomotives on the point to drag this coal train over the summit.
Insiders rarely say the full phrase on the radio, preferring to just call them diesels, units, motors, or power.
Related terms
Traction motor
A heavy-duty electric motor geared directly to the axle of a locomotive or transit car.
Electric locomotive
A locomotive that draws its propulsion power from an external electrical source.
B-unit
A cabless locomotive designed to provide extra pulling power while controlled from a lead engine.