Although other versions of boiler have been tested both latterly and historically with steam locomotives, their utilization did not become prevalent, and the firebox fire-tube boiler has been the overriding source of energy in the era of steam locomotion from the Rocket in 1829 to the Mallard in 1938 and further.
A steam locomotive with the boiler and firebox exposed
The steam locomotive, when fired up, normally uses a steel firebox fire-tube boiler that comprises a heat supply to the rear, which creates and maintains a head of steam inside the pressurized partly water filled region of the boiler to the front.
The heat supply, comprised inside the firebox, is the energy given out by the burning, normally of a liquid or solid fuel, with the by-product of hot burning gases. If coal, wood or coke is employed as the burning substance it is introduced via a door, normally by a fireman, onto a set of grates where ashes fall away from the combustion fuel. If oil is employed a door offers for controlling the air flow, cleaning or maintenance of the oil jets.