This would be realistic as its the main method of tank transportation on land in the real world.

This would also allow you to produce tanks and move them from siege to siege rather than building a flat town with nothing but tanks in it near every fort as every town is suppose to actually be a town according to rule 4.1.

Yes you would need to allow terracotta on trains (if its possible to make it non blast resistant when on a train this would be perfect.) However an armored and unarmed train is not going to be particularly useful for anything as it can only move in a line and stopping it is as easy as breaking the rail.

For the tanks guns you could either allow them on the train but not able to fire like on an aircraft carrier with fighters. or you could just allow the blocks not the gun and we can just shift part of the cannon with a piston.