There are no roads and railways on the other maps. Their routes have to be edited by hand in the terrain editor and that's a major undertaking for an MA map. Any new MA map has to incorporate them before it can be submitted though. There are no supply stations at the ends of the roads like there were in AH2.
Bustr has sneakily hidden the truck convoy's road for each field on Riftval amongst several other non-truck roads, so its not obvious where the convoys will be found.