Tarped-sided auto-carrier trailers.
We know the see-through brass or etched versions are cool but really... those gorgeous frames are pricey! Fortunately for us, there is a prototypical version that is not see-through and it's perfect for a paper model! Check it out!
The solid Trailer tandems are not included but are easy to find as 3D printed on shapeways.com or other plastic or resin manufacturers in both N and HO.