Lay down some track, click forward, and sit back and enjoy the show as a happy black train navigates the path you created. You control the speed and direction. I think I can, I think I can! Be sure to lay the track in the same sequence as you want the train to travel. Train Set works by recording the x and y coordinates of each click as you lay the track. Train Set is admittedly bland as is, but could serve as an excellent starting point for any sort of tile-based game such as Sim City or an exploration of cellular automata.
