Greetings,
If you're looking for some US version dry bulk trailers check out Shapeways.com and see BAZTrains' design for one with round hatches on top and one with oblong hatches. Barry is a great designer and is willing to create something for you as well. These are reasonably priced. They will need painting and perhaps some additional details (to your taste). They are made as ordered and take about two weeks to create in Shapeways 3d printing process. Mine are due in about a week. I'll post pictures when they arrive.
Barry is an Englander and models in HO (US roads). He's a design engineer and his pieces are well conceived, with strong structures and highly detailed considered the material used in these 3d printings.
Bill