I think it is something to do with the trucks/trains getting confused, but I could be wrong about that. As for the new version, I have as much knowledge about the new terrain as any other player.
I think having the TE build road/railway bridges over water would add some extra visual appeal. However there is another reason for doing it. An MA map designer could put a river near a base as a barrier for GVs and then route the supply road across the river. The bridge would then make a good choke point for GV action. GVs would need to be aware of the truck convoy's timing though.