Slab cars, as James mentioned, Baldylox has a 3D one, but the issue is how to hide weight in them. Mike who had them commissioned, was trying to work with Peter at NSK and Brett at Baldylox to do a "hybrid" car, were Brett would design a shell that fit over a sill that NSK made that would hold trucks/couplers.
When I talked to him a few weekends ago, it doesn't seem like there has been much progress on that