The Atlas 40" bridge is too long with the curves leading into it, but something in the 30" range would fit.
Here are some samples that I found, so I can't vouch for the accuracy. The first is an under arch style, the middle is supposed to be a Lionel double truss bridge and the third is supposed to be an Atlas single truss bridge. Here's a link to Lionel's single truss bridge that is 26 1/8" long, 7 1/2" wide and 12 1/2" tall. Obviously, you'll need to figure out how to support it.