If you need to span a longer stretch on a bridge with wooden piers, you can use steel beams like in the photo below. You could also use steel beams to solve a clearance problem beneath.
Brent
"All of the world's problems are the result of the difference between how we think and how the world works."
Sorry, Phil. The "rectangle of support" (first time I ever heard this term in 30+ years of bridge design) has nothing to do with it. Real curved bridges want to roll over because the top and bottom flanges or chords of the girders or trusses are curved, and the forces in them want to push inward or outward because they push at an angle. Even if you stay within the rectan gle, the fact that the bridge bends downward causes forces in the girders that want to make it roll. It's the curved members that are the problem. The loads from extremely heavy locomotives make it impractical to build curved bridges like they do for highways.