Oops! That car's trucks are on span bolsters! I suspect I've been going down the wrong path.
Finally found the H.D. flat car list on pages 739-780. No cars with a capacity of 526,100 are listed in July, 1948. S.P. had 4 FM's with a capacity of 400,000 lbs; C&NW had 2 FG's with a capacity of 400,000 lbs; CMStP&P had 2 FG's with a capacity of 399,000 lbs; NYC had an FM with a capacity of 388,000 lbs and and 2 more with a capacity of 390,000 lbs.
FM's are described as "Ordinary flat car for general service. This car has flooring laid over sills and without sides or ends".
FG's are described as "Flat or gun truck car for special transportation of heavy ordnance or other heavy commodities".
These descriptions do not suggest depressed centers or span bolsters.
As for FD's (i.e., depressed center cars), the heftiest ones I find are RDG, 5 cars, 275,000 lbs; Erie, 5 cars, 270,000 lbs; WM, 4 cars, 268,000 lbs; NYC, 3 cars, 250,000lbs; NYNH&H, 3 cars, 250,000 lbs; N&W, 1 car, 250,000 lbs; PRR, 10 cars, 250,000 lbs; and MP, 2 cars, 250,000 lbs.
I can't find anything in the 1948 O.R.E.R. that references cars with span bolsters. Is it possible that there is a special classification that I haven't found? Note that all the cars listed above are Class 1 railroads. No U.S. Government agencies and no private owners. But the very light color scheme (possibly aluminum) suggests a private owner.
I have no idea whether I've shed any light here.
P.S. A person familiar with the B&O bridge over the AC&Y at Medina suggests that the load probably was not too wide or tall for that underpass. If I'd been in charge, I think I'd have routed it A&BB to Erie at Barberton, Erie to NKP at Lima, Ohio, then NKP to St. Louis. But the seemingly logical routes weren't always used, and it was wartime after all.