the boiler

Steam locomotives are 'external combustion' machines, meaning that the power to operate the cylinders is obtained from steam created in an exterior boiler.


The boiler is the heart of any steam locomotive and is by far the most expensive single part. There are two main components; the barrel and the firebox. The firebox is double skinned so that the metal is cooled by the water contained in the boiler. The two skins are held in place by numerous stays which retain everything in its place. Most importantly, water must always be maintained over the crown of the firebox. This is not easy as the locomotive may at first be climbing, so the water may be over the crown, but when it descends, the water will  move to the front.

The water level over the crown is measured by 2 gauge glasses. The striping behind the gauge glass, which is protected by heavy glass, reverses behind water so the level can be easily seen.

The bottom of the firebox is fixed to the thick foundation ring which has high mechanical strength.

Near the bottom of the firebox are  laid the fire bars which are usually made in cast iron. An ash pan is fitted to the foundation ring which also usually has doors which control the draft through the fire bars. Some engines are able to spray water into the ash pan to cool the red hot cinders in order to reduce the risk of fires.

a small boiler on its side showing the foundation ring

inside a large firebox

In the crown of the firebox is fitted at least one fusible plug. This is made of a metal of a low melting point so that if the firebox crown becomes uncovered, it will melt and allow high pressure steam and water to flow onto the fire to extinguish it.

The boiler barrel is attached to the firebox and many tubes are fitted into it from one end to the other.

front tube plate of small model boiler

The hot gasses pass through the tubes which heats the water in the barrel. A smoke box is attached to the front of the boiler which is kept as air tight as possible. The used steam from the cylinders is directed to a jet below the chimney. This draws the gasses through the boiler tubes. In addition, a jet of steam can be released by a control in the cab (the blower) which will also draw the fire.

Inside a locomotive smoke box showing blast pipes from cylinders under the chimney and the blower tubing. The distance between the chimney and the blast pipe is important.

At the top of the boiler barrel is the dome. Inside is a valve (regulator) which is controlled from the cab. This collects the steam and directed it through high pressure pipes to the cylinders. The dome is important as it allows the steam to be collected as far from the water as possible.

Safety valves are also fitted to the top of the barrel. These are adjusted to open when the steam pressure reaches a preset level to prevent an explosion of the boiler.

The boiler has to be regularly flushed to remove sludge, so at the bottom of the firebox, close to the foundation ring, are washout plugs. These can actually have a tap or a large hex nut head.

super heat

Boilers of greater efficiency use superheating. The steam is passed through additional tubing where it is dried and further heated.

Boiler pressures vary between on average, 120 psi (pounds per square inch) and 250 psi. (some idiot tried building an ultra high pressure engine called 'Fury'. Needless to say, it blew up). Maintenance is onerous. The water to be used has to be analysed and the correct boiler treatment added to prevent scaling and corrosion. In this harsh environment corrosion does inevitably take place. The boiler is usually tested under pressure once a year and every ten years is removed from the locomotive to be thoroughly examined and re-tubed.

Most commonly, boilers are fired using steam coal, which is soft and expands when burning. Oil firing is also used and some locomotives have operated burning wood, bagasse and even peat. One of the less pleasant tasks is that of cleaning out the boiler tubes. This can be done by a special wire brush attached to a long rod or by high pressure steam wands.