A box motor, in railroad terminology, is a self-propelled boxcar

Boxcar

A boxcar is a railroad car that is enclosed and generally used to carry general freight. The boxcar, while not the simplest freight car design, is probably the most versatile, since it can carry most loads...

, normally powered by electricity and running on an interurban

Interurban

An interurban, also called a radial railway in parts of Canada, is a type of electric passenger railroad; in short a hybrid between tram and train. Interurbans enjoyed widespread popularity in the first three decades of the twentieth century in North America. Until the early 1920s, most roads were...

 railway or a streetcar line. Many box motors were converted from passenger cars on the systems that ran them, with the seats and most of the windows removed and large freight doors fitted. They were generally used for express and less-than-carload freight.