Leo Lichtman wrote:

I watched a video about steam locomotives the other day. The top of the
firebox is normally below the water level, but if the water got low, this
steel plate got uncovered. This would cause it to get red hot, and a boiler
explosion would result. This would not be prevented by a relief valve or a
pressure gauge. The sight glasses were the only protection.

Not just locomotives. Steam tractors have
the same design and issue.

The explosion usually does not occur while
the engine is stationary. Its the lurching
start or stop that splashes a quantity of
water onto the red-hot steel, flashes it to
steam and causes a pressure surge.