Bob, Ted and Hot Water,
You are all correct in your analogies of the true X29 box car's shape and size and that Atlas 'O' makes a truer X29 box car, in all aspects, than most others.
As, I have previously posted, that I model the PRR and I have researched the various freight cars of the PRR, to agree with you Gentlemen.
Ted, you have some collection of extensively, researched reference materials.
Ralph