Rather late in the steam era, C&NW converted hundreds of its "USRA clone" SS boxcars in 1954-55. Reduced capital expenditure was probably important to the North Western, and several groups were sold and leased back as part of the conversion process. Presumably, even the lackluster C&NW management could read the writing on the wall well enough to know there wasn't a big future in livestock traffic.
Santa Fe converted a lot of panel-side boxcars, post WW2 IIRC, and UP S-40-10s and -11s were conversions in the '30s and '40s.
