When you see old steam locomotives, you’ll notice that the main drive wheels are connected by a horizontal bar. The steam engine piston attaches to this bar and changes a back-and-forth motion into a circular motion. The bar causes the second wheel to turn exactly the same way as the first wheel.

In the second diagram we see a problem that could arise where the second wheel turns in the opposite direction. If the train stops with the bar centered vertically on the wheels, it could start in this bad situation. What prevents this situation from occurring in real life?