I received the Unofficial LEGO Technic Builder's Guide [link]
for Christmas this year. This bridge design is based off the Warren Truss shown in the book.
This design is highly resistant to vertical compression and tension from either side. It is somewhat vulnerable to twisting forces from either end, however. I think I may be able to solve much of that by adding some sort of X-shaped braces in the center.
I have actually brick-built this specific model, but I want to extend the design out to at least six track lengths. I only have the beams and pins for the two track length segment shown here. My goal: match and eventually beat the length of another bridge that someone else in my LUG has built.
Another downside of only having the parts for this short segment is that I can't test the design properly. I think I may be able to manage if I abandon the red color scheme for the other two 2-length segments for testing purposes, then buy all the parts I need off Bricklink.