Standard terrain setup is a 10-mile long track (rail for strats and HQ, and roads for bases). Look for the little shack near a base and you can trace the route back to the starting point. I haven't timed it out, but I think it's something like every 15-20 minutes a supply train/convoy starts heading toward it's destination. By destroying the supply vehicles, you are increasing the resupply time for its destination.
Every base/strat/HQ has a supply line. Go find 'em and strafe away!
