When did wood boxcars with metal fishbelly underframes first come into common use?
Right around 1900. People started experimenting with them by the 1870s, but it wasn’t until the 1890s, when the Pennsy and Harriman cartel started cranking out outside filshbelly sill boxcars in volume. By 1910, most new designs moved the sills to the center, and by 1918 with the USRA double sheathed box was introduced, they were in widespread use for 40 ton rated boxcars and reefers.
IIRC, from about World War I on, a number of railroads upgraded older wooden cars with fishbelly underframes, and such cars were still in use after World War II.
Dan,
Only REALLY tight roads ever did that. One of them was your own Alton, which rebuilt many 36-foot wood cars with steel (straight) center sills. They still retained some of their trussrods however (there’s one preserved in Monticello, IL).
By the time of the USRA double sheathed boxcar, fishbelly underframes were becoming overkill on cars, as engineering and metallurgy improved. There were many single sheathed cars (generally stronger than DS cars) built with fishbelly underframes, which produced a virtually indestructable car! Lots of THESE cars were rebuilt with steel sides and raised height after WWII, and ran into the 1970s (the NYC rebuilt THOUSANDS of cars like this)