This is a fun game! The controls were nice and clear, and there were some cool puzzles.
A few suggestions:
I accidentally clicked on the start tile which led to me hitting monsters again and needing to restart the puzzle. Maybe you could add a check that if all items on the rail are picked up, then it shouldn't go around or that it only should go around once.
Pressing m/r let me place down monsters and rocks which I think you probably meant to remove.
Removing all rails after a point by clicking on a rail partway along the track could make it quicker to rebuild long tracks.