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"Rich, I don't think any flatcar ever built had an underframe that
looked like Athearn's. They put fishbelly underframes under their
gondola and flat car -- cars that had fishbelly sides for strength!"
Are you are stating that flats with fish-belly side sills weren't equipped with fish-belly center sills? If so, you are incorrect. The vast majority of flats, regardless of the length, built from the 1930s forward were designed with fish-belly side sills and center sills. Flat car designs with straight side sills and fish-belly center sills were vulnerable to twist which weakened and cracked the the side sills which were usually constructed of 12" or 13" channels, thus the movement towards the fish-belly side sills.
While it may be true that the center sills and underframe configuration may not have looked like Athearn's, it is more accurate than a straight center sill coupled with fish-belly side sills. The original Revell underframe was cast metal and was used on both the flat and gondola. I don't remember if it was a fish-belly design, only reason I still have mine is they were both part of my original train set that I have saved for prosperity, but not modeling.
Good luck to Scott, with some effort he'll probably come up with a nice model.