I thought everything about this was really cool & original, really great implementation and awesome & original idea of having a sort of tower defense but on a moving train.
I think if you were to go forward with this idea, the map should be an actual circuit that maybe you could modify or is just what the "level" is, and when you kill enemies, they have a chance of dropping pieces or you can just outright buy it at a station that the train gets to.
Anyways, great job!
Would appreciate your thoughts on ours as well!