mother
A locomotive which provides electrical power for a slug.
mother: the powered unit in a slug-and-mother pair
In rail switching and yard operations, a mother is a standard locomotive equipped with traction motors and a full electrical system that couples to an unpowered slug to form a working pair. The slug, which has no engine or prime mover, draws electrical power through the coupling from the mother's generator or alternator. This arrangement lets a railroad operate two coupled units with the fuel consumption and maintenance cost of running only one.
The mother locomotive maintains normal control of the pair. Its engineer operates throttle, air brake, and dynamic brake for both units. Electrical connections through the coupling transmit power to the slug's traction motors; the slug has its own trucks and wheels but no independent propulsion. The combination produces greater tractive effort than the mother alone would generate, which is why this setup became standard in rail yards that handle heavy cutting and spotting work.
Not all locomotives work as mothers. A mother must have adequate electrical capacity, typically a generator or converter system rated to supply both its own motors and the slug's. Older steam locomotives cannot serve this function. Modern diesel-electric yard engines and some road units are built with this capability, though the mother-slug pair is primarily a switching-yard tool rather than a mainline setup.
The slug itself is essentially a cab-less shell: trucks, wheels, traction motors, and enough receptacle hardware to receive power. Slugs are cheaper to purchase and maintain than full locomotives, and a yard can own more slugs than mothers, then couple them in rotation. If a mother needs servicing, the slug waits for another mother to pair with. This flexibility made slug-mother operations economically popular in freight yards from the mid-20th century onward.
The name follows the industrial convention of treating the powered unit as the primary actor (the mother) and the dependent unit as secondary. The term remains current in North American rail yards, though electronic monitoring and remote-control switching have gradually reduced the need for heavy slug-and-mother pairs in some modern facilities.