Basic Components

Boiler

The typical steam locomotive employs a horizontal fire-tube boiler with the firebox at the rear. The boiler projects slightly into a cab, which shields the locomotive operators from the weather. At the front of the boiler is the smokebox, with chimney (US: "smoke stack") protruding from the top. Steam is collected from the top of the boiler, either in a perforated tube fitted above the water level or from a dome. This system reduces the risk of water entering the cylinders.

Cylinders

The steam passes through a throttle (known as the "regulator valve"), then into the cylinders of a reciprocating engine. The pistons drive the driving wheels directly through a connecting rod (US: "main rod") and a crankpin on the main driver. The valves of the engine are controlled through a set of rods and linkages called the valve gear; this gear is adjustable to control the direction and cut-off of the valve gear. The cut-off point determines the proportion of the stroke, during which steam is admitted into the cylinder; for example a 50% cut-off admits steam for half the stroke of the piston. The remainder of the stroke is driven by the expansive force of the steam. Careful use of cut-off provides economical use of steam and, in turn, reduces fuel and water consumption. The reversing lever (US: "Johnson bar", or "screw-reverser" if so equipped) which controls the cut-off therefore performs a similar function to a gearshift in an automobile. Exhaust steam escapes the engine through the chimney, via a nozzle called a blastpipe. This action creates a draft through the fire grate according to the steam exhausted. The blast of exhausted steam produces the familiar "chuffing" sound of the steam locomotive.

The pistons are double acting, with steam admitted to either end in turn. In a two-cylinder locomotive, one cylinder is located on either side of the locomotive. The cylinders are set 90 degrees out of phase with each other. During a full rotation of the driving wheel, steam provides four power strokes per revolution; that is to say each cylinder receives two injections of steam. The first stroke is to the front of the piston and the second stroke to the rear of the piston; hence two working strokes. Each steam stroke delivers a quarter turn of the wheel. Consequently two deliveries of steam onto each piston face in two cylinders generates a full revolution of the driving wheel. The driving wheels are connected on each side by coupling rods (US: "connecting rods") to transmit power from the main driver to the other wheels. At the two "dead centres", when the connecting rod is on the same axis as the crankpin on the driving wheel, it will be noted that no turning force can be applied. If the locomotive were to come to rest in this position it would be impossible for it to move off again, so the cylinders and crankpins are arranged such that the dead centres occur out of phase with each other. This precaution is unnecessary on most other reciprocating engines (such as an internal combustion engine) which are never expected to start from rest under their own power, and employ a flywheel to overcome the dead centres.

Frame

The boiler rests on a frame, to which the cylinders are mounted and which in turn rest on the axles. The driving axles are mounted in bearings which move up and down in the frame. They are also connected to the frame via leaf springs or, less commonly, volute springs and linkages which allow axles some degree of independent movement over bumps in the track. Many locomotives have leading bogies (US: "trucks") to guide them into curves and/or a trailing bogie to support the firebox weight.

British locomotives usually have plate frames (made from steel plate) while American locomotives usually have bar frames (made from steel bar) or cast steel frames. Each method has significant advantages and disadvantages.

Fuel and water

Generally, the largest locomotives are permanently coupled to a tender that carries the water and fuel. Alternatively, smaller locomotives carry the fuel in a bunker attached to the cab, and the water in tanks mounted on the engine, either beside the boiler or on top; these are called "tank engines".

The fuel source used depends on what is economically available locally to the railway. In the UK and parts of Europe, a plentiful supply of coal made this the obvious choice from the earliest days of the steam engine. Wood-burning engines were found in rural and logging districts in Europe and in the U.S.A. well into the 19th century. Bagasse, a waste by-product of the refining process, was burned in sugar cane farming operations. In the USA, the ready availability of oil made this a popular steam locomotive fuel; the Southern Pacific, for example, went directly from wood to oil. In Victoria, Australia after World War II, many steam locomotives were converted to oil firing. Also in Australia, waste oil fired the West Cost Rail's regular 4-6-4 steam passenger service during the late 1990s. German, Russian and Australian railways experimented using coal dust to fire locomotives.

Water was supplied at passenger stations and loco depots from a dedicated water tower connected to water cranes or gantries. In the UK and the USA, water troughs (track pans) were provided on some main lines to allow express trains to replenish their water supply without stopping. This was achieved by using a 'water scoop' fitted under the tender; the fireman remotely lowered the scoop into the trough, the speed of the engine forced the water into the tender, and the scoop was raised again once the tank was full.

Crew

A crew of at least two people is required to operate a steam locomotive. One, the driver or engineer, is responsible for controlling the locomotive and the other, the boilerman or fireman, is responsible for the fire, steam pressure, and water.