I like to think of this as a tower defense game, but it is much more interactive, which I really is awesome. I do have some suggestions for this.
I feel like there should be multiple guns that you could use. For example, you could start with just maybe like a colt with 8 rounds, then you could work up and buy more guns, I think that you should buff the amount of money you could get like you can get like 10-50 dollars per soldier killed. I think that the minigun should be the most expensive and to balance it out you should make it mounted on the back, and it can only shoot in one direction, as it is mounted. I think to add on to that, you should make the soldiers only run on the train tracks or just make them not go through buildings. I also think there should be more train add-ons that you should be able to buy. Overall I think this is an great game that is very unique!