The firebox is the chamber in a steam locomotive where fuel is burned to heat the boiler water. It is the starting point of the locomotive’s heat-and-steam cycle.
Coal, wood, or oil burns in the firebox. In a solid-fuel locomotive, grates support the fire while air enters from below; ash drops into the ashpan. The hot gases then pass through the boiler’s fire tubes, transferring heat to the surrounding water before continuing toward the smokebox and chimney.
The firebox is usually surrounded by water on its sides, top, and rear, which creates a large heating surface. Its lower part is arranged differently depending on the fuel: solid fuel requires grates, while oil-fired locomotives use a firing pan and burner. Brick arches and firebrick lining helped improve combustion and protect metal surfaces.
The firebox is not the same as the smokebox, where exhaust gases collect, or the ashpan, which catches combustion residue. Nor is it the cab: the cab houses the crew and controls, while the firebox is the intensely hot working chamber immediately ahead of it.