Bridge Layers, I know this comes up alot...

[scorpio_rocks]

an alternate method, which "works" in game, is to use an amphibious vehicle with a large carry capacity and "park" it in the river/marsh, etc and use it as a ferry/bridge.

examples in game include:
Israel #377 "Timsakh" (French Gillois ferry)
Russia #983 GSP Ferry

Your vehicles can then drive up to the river/amphib, load onto it and unload on the other bank. Not as fast as driving down a road but I'm guessing a tank never thunders over a temporary bridge anyway...