Doodlebug or hoodlebug is a nickname in the United States for a type of self-propelled railcar most commonly configured with both passenger and freight (often dedicated baggage, mail and/or express, as in an unpowered combine) compartments. The name is said to have derived from the perceived insect-like appearance of the units, as well as the slow speeds at which they would doddle or "doodle" down the tracks. Early models were usually powered by a gasoline engine, with either a mechanical drive train or a generator providing electricity to traction motors ("gas-electrics"). In later years, it was common for doodlebugs to be repowered with a diesel engine.