Not sure if this was in the last version, but I really liked how you animated the front of the train.
I think the panicking witch in the last train car before the boss might be bugged -- don't think it healed me.
For the highlighting mechanics thing, you reminded me of this video that talks a lot about how to introduce new mechanics (You might have seen it before). I feel like the usual order of things is:
- Introduce new obstacle or enemy in isolation, and in such a way that they can't immediately hit you.
- Throw multiple of the enemy at the player, or put the enemy in a context where they show all their other abilities
- Mix previous enemies and the new enemy together
Only reason I mention this is because you put three screens of isolated new enemies at the beginning. This does the job just fine, but you've used up most of your "novelty" in the first few screens. Might feel better paced if it was more spaced out (Though this would also make things much longer since you've got a decent variety of enemies).