Boiler

Boiler is a large cylindrical water tank onboard a steam locomotive. It has a cluster of tubes going through it, connecting the firebox at the back with the smokebox at the front. Hot air and smoke from the firebox go through these tubes, heating up the water inside the boiler, eventually causing it to evaporate to steam. Being a compressible gas in a pressurized chamber, steam accumulates in the boiler, increasing the pressure. The boiling point temperature of water depends on the amount of pressure in the boiler. The higher the pressure, the higher the boiling point. As the steam in the boiler cools down, it will condense back into water.

Most steam engines have a safety valve that will release excess steam in case the boiler is overpressurized. It is advised to have just as much pressure in the boiler as needed to operate the engine, and not more, as the excess leads to wasting fuel.