Steph: You're close to the truth, but if any steam comes in contact with the surface of the firebox you still have a BANG! The temperature of the fire is hot enough to melt the top of the firebox (the crownsheet), so the top of the firebox must at all times remain under liquid water, with steam forced to remain safely above it. Steam can get as hot as the fire itself, and if there's steam on top and fire below, you will have a crownsheet explosion.

I recall reading about one famous occasion in Virginia when the crew of a C&O Allegheny 2-6-6-6 thought they could make it back into Hinton yard without refilling on water, and only made it to the approach of the yard before the crownsheet went dry. The explosion was like a tonne of TNT going off and completely destroyed the entire locomotive and several of the coal hoppers behind it.

So remember, always top up the tender when you have the chance...

Jurgen