I recently completed a design for a Trail King belly dump trailer, and am in the process of validating the actual printed results from Shapeways. I liked this trailer for its open frame design, and because belly dump and side dump trailers would make great models for N scale construction scenes. My specific model will feature 3 axles, and steel rims, which I modified from my original aluminum rim design used on my Wilson grain trailers.
Here is the 3D rendering on my Shapeways shop. I haven't made it available for ordering, since I want to validate my design, and produce a finished model first. This model has a lot of interesting design features that should result in a neat N scale model. My Shapeways Shop: http://www.shapeways.com/shops/NScaleAddict
I shared in an earlier post that I had a tarped version of my 43' Wilson grain trailer in design. Well, I finally completed it, and I have an order on it's way. I'll share the final result in the near future. This is the 3D rendering on Shapeways, and again, not available for purchase just yet.