Transfer runs are trains that move cars from one freight yard to another within a large terminal area. Early in dieselization, locomotives were built specifically for such service, called transfer locomotives, which demanded a lot of pulling power but not much speed.
Transfer units generally had two high-horsepower (for the time) power plants sitting on one elongated frame, many driving axles, and low-speed gearing, but often lacked multiple-unit controls and dynamic braking. Besides transfers, they worked other similar jobs, such as helper service.
Locomotives with these characteristics were built in switcher, road-switcher, and center-cab configurations. Switcher-type transfer units were of the "cow-and-calf" design, in which two four-motor locomotives, one without a control cab, were built as a drawbar-coupled pair; the set always worked together as a single entity.
Many of the earliest six-motor road-switcher-style diesels were transfer units.
The dual-engine center-cab design is the one perhaps most often associated with transfer service, as most big center-cabs were built for such use. EMC, St. Louis Car, and GE collaborated on a single eight-motor center-cab unit in 1936. Between 1946 and 1954, Baldwin built 70 six-motor jobs, and Lima-Hamilton 22.
Only one survives, a Minneapolis, Northfield & Southern Baldwin at the Illinois Railway Museum.