I really enjoyed this game, it was great! I didn't know I needed a train tower defense game.
As some folks may have mentioned, at times it can be hard to locate the enemies at mid-range. I feel like the gold economy was a little difficult at first (the sellback price seemed very low), but by the end I felt like I had enough.
Not sure if it would fit or not, but I'd love to be able to ram the enemies with the train, and maybe a way to repair the towers (if it weren't a set number of levels that is). I think getting a quick overview of the railroad tracks/map might help in the early game too. Of course, these are just my opinions, so do with them what you will.
Regardless, this was a fun game. Good job and good luck!