As you note, careful attention to the water level in the boiler is a must. Two methods are used to determine the water level. The first as previously described in this thread is the sight-glass. The second method is by means of the tri-cocks, which are three valves which are spaced at different levels on the backhead of the boiler. When opened, the top valve should produce steam, the middle valve should produce a mix of steam and water, and the bottom valve should produce water. If this isn’t the case then “adjustments” must be made. The use of both methods should be done as a check against the other. If one were to enter the cab of a steam locomotive and observe a sight-glass devoid of water, and a bottom tri-cock blowing steam, one’s first course of action (besides running away) would be to kill the fire by closing the firing valve on an oil burner or dumping the grates on a coal-fired engine. The key point here is that the observer is ignorant of the time elapsed since the water disappeared from view. Otherwise, if the fireman saw the water in the glass disappear as the locomotive nosed over the hump of a grade, the course of action would be to add water. Of course a good fireman would know the territory and know his engine, and he wouldn’t allow that to happen. The best way to empty the boiler would be to open the blow-down valve, which is located at the bottom of the boiler. Under normal circumstances this valve is used to clear sediment from the boiler. Just as a point of discussion, I wonder if dumping the water/steam would be good practice. The water temperature in a boiler, which is working 200 psi, is about 380 degrees. As one lowers the boiler pressure, more of the water is turned to steam and thus would lower the water level. It might be as just good to let the boiler cool slowly and avoid the additional stress induced by dumping the water.