Algodoo

5 gear, spring engine powered train;

Press R and then S to start the engine.
Gears are selected by pressing 1,2,3,4,5, but they are toggled - make sure to deselect the gear you're currently in before switching to another.
(You can see which gear you're in by the brake pads that show up on the corresponding hinges)
Z activates the brakes.
To stop the engine, press R, 1, and 5.
To go in reverse, you have to fiddle a bit with holding down S, to get the engine spinning the other way.