Sirenwerks and Jeff: Republic Locomotive Works has a kit for that plywood-side chip car. It was designed for MT 70-ton trucks, because that was all that was available, but Atlas 100-ton will fit, although you may have to do a little carving on the metal floor for wheel clearance. It is designed for GHQ brass ladders.
Disclaimer: I did the original masters, MANY years ago, because I needed a few. I haven't bought any since then, so I don't know how much they are, or how he's packaging them now. Back then it was just the body kit, and the purchaser was responsible for other stuff.