I'll give an equally accurate and slightly more useful answer. You'll have to use models.
http://mtm2.com/%7Emalibu350/scraps/BridgeRamps.htm
Note: Remember that the collision box on a model is a rectangular prism. For a bridge that doesn't look like a rectangular prism (ones with supports or railings) you will need to make the bridge no-collide and then use invisible (purple in traxx) boxes to set the collision surfaces. (Like the checkpoint flag poles in traxx school)
Rectangular prism:
<img src="/%7Emalibu350/scraps/images/trak/RCbridge.jpg">
A bridge that would require 3 purple boxes (one for the road surface, and one for each railing):
<img src="/%7Emalibu350/scraps/images/trak/C2overps.jpg">
Also, one other trick. Computer trucks don't collide with purple boxes. So they will fall through the bridge. Somebody confirm this - or let me know a workaround?
So really I've given you more information but it's not that useful till we get this last piece of the puzzle unless u plan on only racing this track in multiplayer.