A box motor is a self-propelled boxcar used on a streetcar line or interurban railway. Named for its angular appearance, the box motor, like the self-propelled passenger car, runs on electricity that comes from an overhead wire, a third rail or some combination of both and is supplied to traction motors turning the axles. Unlike self-propelled streetcars used to carry passengers, a box motor is used to carry freight. Few, if any, box motors remained in use in the early 21st century.
Not all streetcar lines and interurbans connect with conventional railroads operated with modern diesel locomotives or steam locomotives in the past. Those that do, however, can interchange cars with them and are therefore able to serve their shippers and receivers located in any community reached by a railroad. The catch is that doing so is impossible without a box motor because a conventional railroad’s boxcar or other rolling stock has no means of self-propulsion.
An electric railroad operating partially or completely in a rural area would, in its heyday, have probably carried at least some freight to and from farms along its route, and that would’ve allowed it to use a box motor to deliver commodities or supplies such as bagged feed. The railroad might have run a similarly self-propelled insulated car to collect milk from dairy farms along its route and maintain a cool temperature with ice while transporting it to a central processing plant. It might have operated a self-propelled hopper to carry coal from a supplier to customers or to its own generating plant.
Each of those scenarios presumes that both the seller and the purchaser are located along the same track, because a connecting conventional railroad would have no way to provide electricity to the self-propelled car. The comparable wrinkle in the use of a box motor and one or more unpowered freight cars is that the electric and conventional railroads’ equipment must be of the same gauge in order to interchange. The majority of interurbans use the standard 56.5-inch (143.5-cm) gauge because interchange of freight has usually been a key element in their business, but interchange has rarely been a major consideration, and their gauges can vary.