Checkout the water level in the sightglass!
In “Disneyland Railroad speak”, “two nuts” is the correct level of water to maintain. Of course, on the real train, the water level fluctuates with the amount of heat being applied (converting to steam), the amount of water being injected into the boiler, and even the movement of the train!
A skilled fireman must be able to keep a steady level of water despite all the forces above.