Adds indestructible trains + wagons and they are ignored by the biters (unless the player is on the train!). All except the artillery wagon, but there's an option to enable that.

The reasoning is that artillery is a weapon and should remain destructible, and as for the rest, as soon as a train crashes into enemy forces, it is stopped and destroyed instead of simply plowing right through. If it were to plow right through, I wouldn't make it indestructible.