We just learned about the Johnson Bar that controls the train speed.
Another part of a steam locomotive was the Safety Valve.
This was a valve that could be opened to let off steam if the pressure in the steam engine was getting too high.
![safety valve](https://upload.wikimedia.org/wikipedia/commons/thumb/c/c5/252008_%C4%8CSD_5.jpg/640px-252008_%C4%8CSD_5.jpg)
(from: wikipedia - safety valve)
Kid Facts - Blast from the past: Bearing