Using Google Sketchup, I designed this HO scale model railroad bridge. It's based on the C&O bridge that spans the Ohio River here in Cincinnati. I downloaded a full scale train model from the library and then scaled it down to HO size. This bridge is approximately 7' long which would span approximately 500' if scaled up.
The challenge with this project is finding the right materials to construct it. The larger supports are 1/2" square, the smaller 1/4". Unfortunately, at that size, wood tends to warp over large lengths. Aluminium or perhaps plastic pieces 3D printed would be ideal.