Looking at different circular layouts similar to the following with a single turnout:
I remembered an old shunting Puzzle, actually made on a small wooden foldable plate, tracks are drawn on the board, loco and cars are made of wood.
Here's the rules I remember so far:
Place the cars to their starting positions (bright car=bright space, dark car=dark space, loco in between; adapted to the pic above this means: blue car to blue space etc. ...). The positions of the cars have to be changed by shunting (blue car to green space, green car to blue space). Each shunting possibility is allowed, but the loco is allowed to go only two times via the tunnel and without any car. In the end the loco has to be on its starting position again.
Looking at Ian's Purespring Watercress these rules came up to me again.