The OP800 was a lightweight, streamlined railcar built by the St. Louis Car Company in 1939. Fairbanks-Morse supplied the 800 hp (600 kW), five-cylinder 8 in × 10 in (203 mm × 254 mm) opposed piston engine prime mover. The units were configured in a highly unusual 2-A1A wheel arrangement (later converted to 3-A1A) mounted atop a pair of road trucks, and equipped with a front swing coupler pilot. The aft section was divided into two separate compartments: one was used to transport baggage and the other served as a small railway post office, or RPO (the forward door, located just behind the radiator louvers, was equipped with a mail hook).