The labels are removed from the diagram but I believe the large gears in the middle are on top of a main spring, you can see a bit of the coil on the right. So

The labels are removed from the diagram but I believe the large gears in the middle are on top of a main spring, you can see a bit of the coil on the right. So the diagram shows the mechanism upside-down. At the top is the start/stop wheel. One revolution of that wheel and the lever clicks into the single slot, stopping the motion. Just below that are the 3 travel wheels which are toothed for traction. The rear axle has a gear that engages the large gears on the main spring below. Left of the main spring are the gears for limiting the speed, a kind of governor.