Look at the first image. The top left has a track that's completely unoccupied. That's key. Some levels have 2 empty tracks, which of course makes things easier.
Look at the second image: there are no empty tracks. Much harder.
There's a lot to like about Train Mix. The graphics are good and the puzzles are challenging.
Helpful: suppose you make a move that leads to an impossible situation, a red light flashes in the upper right corner warning you of this fact and offering the undoing of that last move. Great programming. Fun icon.
There are 234 levels. I did 34.
Six Towers Plus