I love the aesthetic of the game, really awesome style!
However I don't know if I did something wrong or missed an instruction but my train seemed to be far slower than the one in your gameplay video. Like I stay at 0.0km for at least 15 seconds (vs like 0.5s in your video), and I have all the time in the world to destroy every gem in the screen (vs your gameplay where the train goes so fast that you can't hit them all). And since the train is so slow the game never offers me to upgrade it (I die before that).
But I think the problem comes from me (I'm playing on a rather-slow laptop), so I must have a lot less FPS. Maybe the train acceleration isn't frame-independant?
But I adore the presentation and the pacing from your gameplay video seems really fun too! Great job!