The train moves one to four squares per tick, and will push aside units it runs into, doing 50 damage. Normally, a unit is pushed off the tracks, but if there's no space for that, it is pushed forward. A longer train can play an important role in the game as it can block off whole parts of the map for all non-flying units.
Cattle run away from the locomotive as they run away from other units, but trains are currently blocked by cattle that stand on the tracks with their heads toward the train. The train will just ignore gold nuggets and bonuses, causing them to occasionally share a square with a part of the train. The train may also push some of these items off the tracks. If the locomotive collides with any other part of the train, it will be blocked and stand still.
At the moment, there are no ITP maps with a train on them.