An obvious safety feature for protecting the crown sheet of a steam locomotive is the installation of lead plugs that melt before the crown sheet gets too hot in a low water situation. When a lead plug melts, steam will blow out of the newly created opening and put out the fire and will also lower the boiler pressure. While in some quarters these plugs are referred as lead plugs, frangible plugs or melt plugs, I am not sure that they are actually made of lead or some other lower melting point metal. When my brother fired in South Africa, all of the engines had lead plugs, and there was a schedule of fines for firemen who dropped plugs. I vaguely recall, although I am not sure about the numbers, that there was a 500 Rand fine for the first plug, a 1,000 Rand fine for the second, and termination of employment after the third plug dropping incident. I am not sure how common fusible plugs were on engines in the USA, but I know that 315 and the K class engines don't have them. I have always wondered why that is.