These 53'-6" flatcars were constructed with a welded fish belly steel frame in contrast to the cast steel GSI frame. The fish belly frame provided an optimal distribution of tension and compression forces from the center of the car to the bolsters. Think of it as a bridge. In addition, fabricating a welded frame was far less labor intensive than an equivalent riveted frame. The welded frame proved extremely durable holding up to rugged service. As these flat cars aged-out due to ARR interchange age limit, they found it's way into many years in maintenance-of-way service. A number of builders constructed these flat cars such as the Pacific Car & Foundry (P.C. & F.), Marine Industries (Canada), Pullman-Standard, Bethelehem Steel, and including railroad car shops such as the Western Pacific.