locomotive built in the United States. Freight cars As part of the USRA, two common boxcar designs were developed: a single sheathed car and a double sheathed car. When the USRA boxcars were being designed there wasn't an industry consensus on which was better, so both were built. Freight car design was still in flux in the early part of the 20th century. As John White points out in The American Freight Car, most cars were really composites, not completely wood or steel, and even after steel cars had become the norm, wood had its uses and advantages. Still, steel underframes had come to replace wood underframes, but as metallurgy improved, there were new designs developed that took advantage of the improved technology. USRA double sheathed boxcars had a fishbelly underframe while the USRA single sheathed cars did not. In general, double sheathed boxcars are like girder bridges, so all the support needs to come from the frame. Those cars need a stronger frame, hence the fishbelly underframe. Single sheathed cars are like truss bridges, with the metal side bracing acting as the main structural support for the "bridge." While some engineers did not trust the steel bracing to support a single sheathed car and ordered fishbelly frames for strength, others valued the savings in weight and ordered cars with simpler frames like the USRA SS design. References Rolling stock of the United States