The motor-generator type of locomotive permits the application of the direct-current series-wound traction motor, with its highly desirable characteristics, to electrifications deriving their power from a single-phase alternating-current trolley. This is of particular benefit in the operation of heavy-drag freight trains on mountain grades. Nearly 30 years of design and operating experience have gone into the development of this type of locomotive. The latest examples are the units recently placed in service on the electrified sections of the Great Northern and Virginian Railways. Noteworthy features in the design, construction, and operation of these units are discussed.