Ok so I've been researching building my own miniature gauge steam loco, and was curious on boiler safety and testing. I came across a fairly catastrophic true story of what can happen if the water gets too low. This is my interpretation of what happened. In East Germany steam locomotives had been used on scheduled services until the late 1980s. In November 1977 approaching Bitterfeld station, a DR class 01.5s normal driver was on a course and had been replaced by a driver not fully experienced with this type of locomotive. Lack of communication between the fireman and this temporary driver resulted in the water gauge not being checked. The result was that the water got so low that it was no longer covering the firebox. In a period of 4 minutes the steel fire box had heated to nearly 800 degrees C, starting to cause the metal to fail. The train entered Bitterfeld station and as it came to a halt the inertia caused water in the boiler to swash forward and then back over the fire box. (Now these locos have 9 cubic metres of water inside and as you know from physics that under pressure a liquid can remain liquid well above its boiling point and become a gas when pressure is released.) The action of the water flowing back over the red hot fire box top resulted in it 1) instantly turning to steam, and 2) causing severe changes to the structure of steel in the firebox top. The result was a massive localised pressure build up over the top of the box, which caused the fire box top to collapse. In an instant 9 cubic metres of super heated water flashed to 3000 cubic metres of steam about 1 billion watts of energy. This blasted through the fire box opening into the cab like a rocket jet nozzle, ripping the entire boiler from its chassis and propelling it like a rocket 50 metres down the station! The 2 crew members were killed plus 7 bystanders in the station. Just shows how a steam engine boiler can be like a bomb. Never underestimate the energy in compressed liquids and gases. See: http://www.vapeur-dampf.ch/la-vapeur-pas-de-peur-mais-du-respect/?lang=en An interesting topic to discuss. Please feel free to add any more storied or examples of boiler mishaps etc!