Let's consider the thermodynamics of this.... Water in a pressurevessel (fancy name for a boiler or sealed tank) boils at a higher temp than 212 deg F. For example, at 200 pounds of pressure, water boils at 386 deg. So, for example, a steam loco boiler contains 2500 gallons of water, under 200 psi (pounds per square inch) pressure, with all that water being at 385 degrees. Now, the way the firebox area is constructed, there is a firebox formed of 3/8" to 1/2" steel plates, supported by 1" dia threaded "staybolts"bewteen the inner wall (firebox) and outer wall (boiler wrapper sheet). Water circulates all around the sides, both ends, and the top of the firebox, aka the "crown sheet" (there are some good diagrams of this on the net, and in books). The fire is burning on grates in the bottom of the firebox, and is at about 2200 degrees. The water on the other side of the firebox sheets rapidly conducts the heat away, and makes steam in the process. Temp at waterside of sheet is about 600 degrees. Steel gets "plastic" and loses its tensile strength at about 1200 degrees. The firebox steel is usually rated at 50,000 psi tensile strength....see where I'm going with this? The vast majority of boiler explosions (actually firebox failures) are caused by low water. If the crew is inattentive to the water level in the boiler, or can't get water in (due to mechnaical component failure or false reading on water gage), then the crown sheet is the first to get uncovered and goes "plastic" as the full strength and heat of the fire takes the steel temp up to and over 1200-1500 degrees. The steel now has NO strength to resist the boiler pressure (remember, it's still at 200 psi), and the crown sheet is forced by the boiler pressure off the staybolts. As this happens (in tenths of a second), the 386 degree water under 200 psi all turns to steam-- the water is way above boiling point, and now there is no more pressure. And, water turning to steam expands 1600 times in volume-- so 2500 gallongs of water turns instantly to 4,000,000 gallons of steam!!!!
The force created is so great that the back end of the boiler (where the cab is) is usually forced off the frame, and the entire boiler is launched into a trackside field, sometimes hundreds of feet. Needless to say, the crew usually doesn't make out too well...
There was a good reason why that in the steam era, the water glass (boiler water indicator in loco cab) was called "the most important window on a train". A crew ignored low water at their peril.
See the US DOT-FRA Accident reports for the 20s through the early 50s for some detailed descriptions of loco boiler "incidents". Really gives a feeling of respect for the power of steam, and for the oldtimers who handled it thousands of times daily for 150 years.