Beginning in 1925, the Southern Pacific built nearly 5,000 40’ outside braced wood boxcars in two classes – B-50-15 and the B-50-16. These cars were built to nearly identical designs, the primary difference being in the ends used. The 3900 cars in the earlier B-50-15 class used corrugated steel ends, while the 1003 cars in class B-50-16 used “Dreadnaught” ends. Both groups of cars used a variety of different roofs, with the Murphy radial roof and the Viking roof being the most common. In fact, nearly all B-50-16 received Viking roofs. Most of these cars were delivered with T-section trucks, though many were later upgraded with “Bettendorf”-style trucks.

Starting in the 1930s the Southern Pacific started a program of re-siding cars in both classes, replacing the wooden sides with steel sheet sides inside of the side bracing. This program continued on and off for many years, and not all cars were rebuilt. The steel sided cars in particular remained in service for many years, with 227 B-50-15 and 25 B-50-16 cars still in revenue service as late as 1970. Because of their antiquated looking construction, they really stood out in a train by that date.

Over the years these cars carried several versions of Southern Pacific paint schemes. Starting in 1955 they were also renumbered into more cohesive groups, the original numbers being in many unrelated groups.