DISASTER!!! on 15F steam locomotive courtesy of fusible plug

You have to watch the video for 3 minutes and 40 seconds to see what happened!
A fusible plug, also known as a safety plug, or threaded fusible plug. It is one such safety device that is commonly used in steam engines. It ensures that in case of water levels recede to dangerous levels, operators will be alerted by a strong stream of steam, allowing them to take action.
A fusible plug is a threaded cylinder of metal, with a tapered hole drilled completely through its length. This hole is sealed with a metal of low melting point that flows away if a pre-determined, high temperature is reached. The initial use of the fusible plug was as a safety precaution against low water levels in steam engine boilers.
Typical locations for fusible plugs include the crown sheet (top of combustion chamber), rear tube sheet (Scotch marine boiler) and front tube sheet (firebox boiler). The location chosen depends upon the boiler design.