just a thought - I've done negligible building, but could this be simulated by greatly increasing the movecost of all the terrain and equally increasing the move rates of all units but the locomotive in the rules.txt?
alternatively, maybe the locomotive could have its movement rate set to 0< mr < 1 since movement rates are floats instead of integers?
a third thought, which i guess would only have a chance of working if you have Multiplayer gold, might be to make a recurring event that lowers the movement every round of the locomotive to simulate having moved n-1 road spaces (n=number of spaces you can move on a road with 1 movement point)?
just trying to think outside the box - any chance of any of those suggestions working?