curve, they are very close. I looked into what the differences were a while back. Here's what Craig Zeni said about them:
The x-29 and the Seaboard express boxcars are of similar height, have similar flat roofs and ends but beyond that the details vary. The Seaboard car is a 1932 ARA design with youngstown doors and a tabbed side sill reflecting the more modern underframe design. I believe [but would have to research] the wheelbase is different, the PRR car having the PRR's own truck design [2D-F8 or 2D-F12] mounted 6 inches closer to the ends of the car.
Look closely at Ed's photo and you'll see that the wheelbases are different. The X29s truck kingpins are 5' from the end; the SAL cars are 5.5', the AAR standard. Doesn't sound like much but that extra 6 inches or lack thereof gives the PRR cars a distinctive look even in HO scale if you're a bit of a freight car geek. The net effect is that the trucks are 12" closer together on a proper X29 which is a bit over 1/8th of an inch.
The 1932 ARA cars can be had from Atlas with the appropriate flat (plate) ends and build up into very nice models. I've done a couple using the excellent Speedwitch decals which appear to be available again.
Craig Zeni
Atlas did offer a X29 car painted in SAL colors (got it) but it lacks the side sill tabs.
SALs ARA 1932 cars (class B6) were numbered 17000-17699 and 18000-18999. They took some of them and converted them into these ventilated cars, 700-754, right here at their Portsmouth Va shops.
Andy Sperandeo made a video on express cars a while back which included a SAL car (around the 7 minute mark):
http://mrv.trains.com/videos/e...ess-train-car-models
He says these cars showed up in Santa Fe trains and one photo in one of my books shows a car in Milwaukee in 1966.
