Managed to beat it, but really enjoyed it!
If you want some feedback ... read on. Or otherwise ignore!
I came up with some of these ideas, and then read through and saw that lots of people thought the same things. Ah well ... I've written it now!
Not all the demons were flagged by the game once they had arrived, I had to drive around looking for stragglers.
I thought gameplay might be improved with just using cursor keys instead of the slider, but I guess the difficulty in controlling the train was part of the design?
Could you include causing damage with the train, i.e. running them over? I initially thought that was the point of the game. The faster you go the more damage, but you might fly by, and minimise the impact your archers etc do.
I think you could "easily" do different levels, or tracks for variety. There is the potential for a much bigger game here.
Optional power ups. Choose to repair your towers or upgrade the train? Wandering mobs that support you?
Also, just a suggestion (which I presume would take a lot of work, so feel free to ignore), some ability to lay tracks where you want.
I will probably play this again though! I love the concept!