need help plz!

[THORgodofHAMERS]

I am making a zombie survival map in the map you start out on a train survivors have armored and are using as a mobile base the map has the train as 4 units linked together and move together like a real train but i need a way to have the train be able to build a turret on top of it so it would go from a blank train to a train with a turret on top plz help!!!