Boiler explosions are deadly! At 300 PSI, the boiling water (and the steam) is over 400F.
Early locomotive boiler explosions (if not caused by collision or derailment) were often caused when the stay bolts suspending the firebox in the rear of the boiler failed due to corrosion, sending the firebox, followed by steam, back into the cab with fatal results for the crew. When the pressure is suddenly released from the boiler, the water is far above the boiling point at atmospheric pressure, and flashes to steam, and lots of it.
It turns out that many stay bolts would fail before the boiler failed, but there was not warning to indicate the stay bolts had failed. To make them safer, stay bolts were later designed with a hollow center open to the outer bolt head, so if the stay bolt cracked (and the weakest part was the hollow section), a tell-tale weep/steam leak would be visible and/or audible during periodic inspections at pressure. When a certain percentage of the stay bolts were damaged, the boiler had to be rebuilt. After each rebuild, the rated maximum boiler pressure was usually reduced for safety, eventually below 200 psi before they were scrapped.
Boiler explosions due to engine derailments and collisions at speed were often fatal to the engine crew (fireman and engineer). When the collision ruptures the boiler, all the boiler water flashes to a whole lot of steam.
-- Andy - Arlington TX