I have a few Atlas Pennsy X-26 single-sheathed, or outside braced, boxcars. These were part of the 1918 USRA program and the Pennsy received 9,900 of the total 25,000 cars built.
Here are some non-Pennsy boxcars I have.
I have a couple of Lionel MoW double-sheathed cars in the correct gray scheme for pre-1954 Pennsy. Unfortunately, the Pennsy never owned any double-sheathed boxcars as far as I can tell.
I have two Lionel non-Pennsy double-sheathed boxcars, but the paint schemes on them were gone by my layout's late 40s era. The Santa Fe had a large fleet of the USRA cars.
The date on the SSW car indicates it was not part of the original USRA order but a clone.