Very good idea!
Love sandbox element of this game, and especially, that you can make it a little broken, exploring some bugged ways to make train go was very funny! Sounds are nice, don't disturb.
Some particle effect from a train would look quite great! You can add some layer of difficulty to the game by making some obstacles, like objects in world, so train will need to go around them. Something like Mini Motorways would fit this game.
But overall, great job! You should evolve this game in something more final, would love to play that more!