Absolutely loved the slider, great idea, it makes the train just hard enough to control. If you're open to suggestions I've thought up a few.
You could add some kind of brakes failure or train crash that happens when a player going at rapid speed tries to brake at full power. (Also having sparks appear when braking at high speed)
A way to restore broken towers, maybe 100 g to arise a broken tower back to 1/5 HP, could be implemented.
Also, a land unit wagon sounds fun, something that spawns a small squad to block and fight ennemies.
Other than that, having a station(s) to buy upgrades from rather than buying upgrades between rounds would allow you to keep the ennemies indefinitely spawning , with small rests periods happening whenever the ennemies' spawns change locations.
Finally, this mechanic would probably change the general flow of the game, but if you intend for it, you could add train and wagon HP + collisions with the ennemies, meaning you can ram them in order to do (and take) damage, and the ennemies cannot go through your ship, instead having to damage it until it is destroyed or the player moves it out of the way.
And that's about it, sorry for the long message and any potential grammatical mistakes. Really loved this game, the market's appearance is well made and the art is fabulous, loved the presence of vegetation rather than having nothing but rock, stones and lava. I wish you luck and really hope your game prospers.