Boiler safety issues

I'd say Peter was correct in his description. A boiler isn't secured to the frames at the firebox end, it's merely restrained from vertical movement. Though if the forces are high enough, it'll easily break the castings or fixings holding it down.

 

Generally, a firebox crown collapse exerts a vertical force on the boiler via a down-force through - and blowing out - the firebars and ashpan. The vertical restraint provided by the firebox slide mounting usually isn't sufficient to resist this and the firebox end of the boiler is propelled upwards out of the frames. The restraining force exists though, and many engines which suffered a crown collapse derailed due to the up-force before the boiler's departure. There are several illustrations in C.H. Hewison's 'Locomotive Boiler Explosions' (1983) David & Charles ISBN 0 7153 8305 and in the 'Trains in Trouble' series.