Welded Underframes: The Prototype

Another one of the interesting and unique features of Union Pacific's steel boxcar fleet, was the utilization of welded, rather than riveted, under frames. Most railroads, including the Northern Pacific, used the standard AAR riveted design, and while underframes are not a highly visible feature of freight cars, I have found it interesting to fully understand the differences between the UP's welded design, and the AAR standard. 

The first series of UP boxcars to use the welded design were the 1937 built B-50-21s, and the design continued in use, on both 40 and 50-foot cars, throughout the 1950s. Although Ted Cullotta shared his modeling techniques of UP's welded underframe in his "Essential Freight Car" series in Railroad Model Craftsmen ("Union Pacific's ACR Boxcars," February 2009), the article did not include any prototype photographs of the underframe. Whenever possible, I prefer to reference prototype photos, and so presented here is a small gallery of images illustrating UP's welded underframe design on a surviving B-50-27 in Washington, and on a B-50-54 in Nebraska. Note the additional reenforcement added under the floor of the B-50-54 when it was modernized, and increased to 110,000 pound capacity.  

Union Pacific (OSL) 910390, B-50-27, Constructed in 1940 (189400-899).

Overview of welded underframe on B-50-27 from B-end.

Overview of welded underframe on B-50-27 from A-end.

Union Pacific 108875, B-50-54 Rebuilt in 1965 from B-50-39 constructed in 1947.

Overall view of welded underframe from B-end of the car, showing the unique crossties and crossmembers.

Overall view of welded underframe from A-end of car. 

Detail of air cylinder and mounting arrangement on the crosstie. 

 Although the underframe is not highly visible on scale models, I find it satisfying to accurately model the entire car, including the underside. For resin, or modified styrene models, Ted Cullotta produced a resin kit of this underframe in HO scale that can be used. However, I prefer working with styrene and liquid cement, rather than resin and CA, so I came up with my own process for building up a styrene underframe that accurately represents the UP prototype. This process will be detailed in a future blog post.