On Jul 15, 11:20*am, Nick Leverton wrote:

Melting of fusible plugs (thus releasing boiler pressure steam into the
firebox)

Although dropping the plugs into the box is indeed "boiler pressure
steam", the plugs are sized small enough to be noticeable from the
footplate, but certainly not hazardous. In one famous boiler explosion
(the Coronation at Lamington in 1948) the plugs had dropped some time
earlier and no-one noticed. The 1962 explosion was very similar - a
Coronation where the plugs had dropped and finally the firebox
crownsheet softened and pulled over the stay heads.