Doodlebug or hoodlebug is a nickname in the United States for a type of self-propelled railcar most commonly configured to carry both passengers and freight, often dedicated baggage, mail or express, as in a combine.

They were popular with some railroads during the first part of the 20th century to provide passenger and mail service on lightly used branch lines at less expense than with a train consisting of a locomotive and coaches with larger crew.

The development of gasoline engines led railroads to seek them as higher efficiency alternatives to steam power for low-volume branch line services at the start of the 20th century.

The 200-horsepower (150 kW) engine on the 55-or-70-foot-long (17 or 21 m) units drove only one set of wheels, and the lack of power and traction, the unreliability of their transmissions, and an inability to reverse, were major limitations.

However, their low operating costs prompted the construction of the Galloping Goose railcars built by the Rio Grande Southern Railroad (RGS) from used Buick and Pierce-Arrow automobiles with a custom-built cargo box or flatbed behind the body.

[11][12] The last remaining Atchison, Topeka and Santa Fe (ATSF) gas-electric doodlebug, M.177, is on display at the City of Los Angeles "Travel Town Museum" in Griffith Park.