Steam locomotive components
This article is a glossary of the main components found on a typical steam locomotive.
The diagram, which is not to scale, is a composite of various designs in the late steam era. Some components shown are not the same, or are not present, on some locomotives – for example, on smaller or articulated types. Conversely, some locomotives have components not listed here.
Details of the componentsEdit
Attached rail vehicle that holds both water for the boiler and fuel such as coal or oil.
|2.||Cab (US+ and UK)|
Compartment where the engineer (US) / driver (UK) and fireman control the engine and tend the firebox. They achieve that using various devices, principally:
Steam powered whistle, located on top of the boiler and used for signalling (by the number and length of notes) and warning of approach.
Rod linking the reversing lever (US: Johnson bar) or reversing lever or wheel (UK) in the cab to the valve gear.
|5.|| Safety valve|
Pressure relief valve to stop the boiler pressure exceeding the operating limit.
|6.||Generator / Turbo-generator|
Electrical generator driven by a small steam turbine, for the headlight and other locomotive lighting.
|7.|| Sand dome|
Holds sand that is directed on to the rail in front of the driving wheels to improve traction, especially in wet or icy conditions or when vegetation is on the line, and on steep gradients.
|8.||Throttle lever (US+)|
Sets the opening of the regulator valve / throttle valve, (31) which controls the amount of steam entering the cylinders, hence the speed of the locomotive. It is used in conjunction with the reversing lever to start, to stop, and to control the locomotive's power output. When the regulator/throttle is closed, an anti-vacuum valve (snifting valve) permits air to be drawn through the superheater and cylinders to allow the engine to coast freely. The throttle is not the only control that can limit the locomotive's power output: during steady-state running of most locomotives, the throttle is usually set wide open and the power output is controlled by moving the reversing lever (2) closer to its mid-point ("reducing the cut-off") to limit the amount of steam admitted to the cylinders.
|9.|| Steam dome|
Collects steam at the top of the boiler (well above the water level) so that it can be fed to the engine via the main steam pipe, or dry pipe, and the regulator/throttle valve.
|10.|| Air pump / Air compressor |
Westinghouse pump (US+)
Powered by steam, it compresses air for operating the train air brake system. The Westinghouse air brake system is used world-wide; in Europe two systems that use the same principle are the Kunze-Knorr and Oerlikon systems. It can be a single-stage or, when larger capacity is needed, a two-stage cross-compound compressor. Vacuum brakes, used historically, do not employ compressors; as a result of their relative inefficiency they are no longer in large-scale use.
Receives the hot gases that have passed from the firebox through the boiler tubes. It may contain a cinder guard to prevent hot cinders being exhausted up the smokestack / chimney. When the regulator is open, the smokebox is where steam exhausting from the cylinders is directed up the chimney, sucking air through the firebed. Components in the smokebox are:
Carries steam to the cylinders.
Hinged circular door to allow service access to the smoke box to fix air leaks and remove cinders.
|14.||Trailing truck (US+)|
Rear bogie (UK)
Wheels at the rear of the locomotive to help support the rear of the locomotive and improve riding qualities – see also Leading wheel (46).
|15.||Foot board / Running board / Tread plate|
Walkway around the locomotive, from the cab front, to facilitate inspection and maintenance.
The strong, rigid structure that carries the boiler, cab and engine unit; supported on driving wheels (43) and leading and trailing trucks (14, 46). The axles run in slots in the frames. Early American locomotives had bar frames, made from steel bar; in the 20th century they usually had cast steel frames or, in the final decades of steam locomotive design, a cast steel locomotive bed – a one-piece steel casting for the entire locomotive frame, cylinders, valve chests, steam pipes, and smokebox saddle, all as a single component. British locomotives usually had plate frames made from steel plate but some end-of-era designs included cast steel sub-frames.
|17.|| Brake shoe / Brake block|
Cast iron or composite material that rubs on all the driving wheel treads for braking.
|18.|| Sand pipe|
Deposits sand directly in front of the driving wheels to aid traction on steep gradients, when starting or when the rail surface is not dry and clean.
|19.||Coupling rods / Side rods|
Connect the driving wheels (43) together.
|20.||Valve gear |
System of rods and linkages synchronising the valves with the pistons and controls the running direction and power of the locomotive.
|21.||Connecting rod / Main rod|
Steel arm that converts the reciprocating motion of the piston into a rotary motion of the driving wheels. The connection between piston and main rod is a crosshead, which slides on a horizontal bar behind the cylinder.
|22.|| Piston rod|
Connects the piston to the cross-head.
Produces the motion for the locomotive from expansion of the steam. Driven backward and forward within the cylinder by steam delivered alternately, in front and behind, by the valve.
Chamber that receives steam from the steam pipe.
Controls the supply of steam to the cylinders. The valve gear, actuated by connection to the driving wheels, ensures that steam is delivered to the piston with precision. Types are slide valves, piston valves or poppet valves.
|26.||Valve chest / Steam chest|
Valve chamber next to the cylinder (24) containing passageways to distribute steam to the cylinders.
Furnace chamber built into the boiler, which produces steam in surrounding water. Various combustible materials can be used as fuel; the most common are coal and oil but in earlier times coke and/or wood were used.
|28.||Boiler tubes and flues|
Carry hot gases from the front of the firebox to the front of the boiler, producing steam from the surrounding water. Flues are larger in diameter than tubes because they contain superheater units.
Horizontal tubular vessel, strong enough to contain high-pressure steam in a harsh working environment; closed at either end by the firebox and tube plate. Usually well filled with water but with space for steam – produced by heat from the firebox and boiler tubes – to be above the water surface.
Pass steam back through the boiler to dry and superheat it for greater efficiency. See Superheater (32).
|31.||Throttle valve (US+)|
Regulator valve (UK)
Controlled by the Throttle Lever / Regulator (8), regulates the amount of steam delivered to the cylinders, which is one of two ways to vary power of the engine (throttle governing). For the other method, see Throttle lever (8).
Provides additional heat – as much as 300 °F (167 °C) hotter – to steam that has been generated in the boiler by sending steam back through superheat tubes located in the boiler tubes (28), thus increasing engine efficiency and power. See also Superheater tubes (30).
Vertical pipe on top of and inside the smokebox that ejects the exhaust (smoke) above the locomotive.
Light on the front of the smoke box to illuminate track ahead and warn of the approach of the locomotive.
Hose for conveying force to train brakes by a differential in air pressure. Contains either high-pressure compressed air or air at lower than atmospheric pressure (vacuum), depending on whether the locomotive has an air brake or vacuum brake system.
Tank for water used by the boiler to produce steam.
Compartment for storage of fuel before being directed to the firebox. When the fuel is coal (and in the distant past, coke or wood), the fireman shovels it manually through the firebox door or, in larger locomotives, by operating a mechanical stoker. When the fuel is oil, it is sprayed into the firebox from a sealed tank.
Supports the burning fuel while allowing the products of combustion – ash and small clinker – to drop through.
Collects the ash from the fire.
|40.||Journal box (US+)|
Axle box (UK)
Housing for the bearing on the axle of a wheel (43).
|41.|| Equalising beams / Equalising levers / Equalising bars|
Part of the locomotive suspension system. Connected to leaf springs; free to pivot about their centre, which is fixed to the frame. Their function is to even out weight carried on adjacent axles, which is especially necessary on uneven or poorly laid tracks.
|42.|| Leaf springs|
Main suspension springs for the locomotive. Each driving wheel supports its share of the locomotive's weight via leaf springs that connect the axle's journal box / axle box (40) to the frame.
|43.||Driving wheels (UK)|
Wheel driven by the piston (23) to move the locomotive. The weight of bearings and coupling rods (20) on the driving wheels is counterbalanced with cast-in weights to reduce "hammering" on the track when the locomotive is under way.
|44.||Pedestal / Saddle|
Connects a leaf spring to a journal box / axle box (40) on a wheel. 
|45.|| Blast pipe|
Directs exhaust steam up the chimney (33), creating a draught that draws hot gases through the firebox (27) and along the boiler tubes (28).
|46.||Pilot truck, Pony truck (US+)|
Leading bogie / Lead truck / Bissel truck (UK)
Wheels at the front to guide the front driving wheels around curves, and minimise yawing at higher speeds. The truck is equalised to the driving wheels (41). The names Pony truck (US+) and Bissel (or Bissell) truck (UK) apply when there are two wheels; the others when there are four.
|47.||Pilot / Cowcatcher(US+)|
Prevents an object on the track from going under the locomotive and possibly derailing the train.
Device at the front and rear of the locomotive for connecting locomotives and rolling stock.
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- Forney 1891, p. 425.
- Forney 1891, p. 416.
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- Glossary of boiler terminology
- Horsepower#Drawbar horsepower
- Power classification
- Rail terminology
- Tractive effort