Main components found on a typical steam locomotive include:
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File:Diagram_of_steam_locomotive_components_(March_2021_version).tif|thumb|center|800px|The main components of a typical steam locomotive. Click or hover over numbers to see names. (Template:Plainlink)
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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 as, or are not present, on some locomotives – for example, on smaller or articulated types. Conversely, some locomotives have components not listed here.
Tender Attached rail vehicle that holds both water for the boiler and fuel such as wood, coal, or oil.<ref name=lindahall>{{#invoke:citation/CS1|citation
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a throttle lever or regulator, which controls the amount of steam entering the cylinders<ref name= Fowler/>Template:RpTemplate:Rp
a reversing lever or (US term) Johnson bar, which controls the timing of the admission of steam into the locomotive's cylinders.<ref name= Fowler/>Template:Rp This is required for two purposes. One is to reverse the locomotive's direction, e.g. when shunting. The other is to enable more fuel-efficient operation when the locomotive is running in a steady state: the throttle is set wide open and the power output is controlled by moving the reversing lever closer to its mid-point ("reducing the cut-off") to limit the amount of steam admitted to the cylinders.
a train brake lever, which controls the application of brakes throughout the length of the train and a locomotive brake lever, controlling brakes on the locomotive only<ref name= Fowler/>Template:Rp
injector valves, which allow steam to force water into the boiler when needed<ref name= Fowler/>Template:Rp
water gauges, which allow the level of water in the boiler to be monitored<ref name= Fowler/>Template:Rp
mechanical stoker controls (when fitted to larger coal-fired locomotives) or oil feed controls for oil-fired locomotives
lubricator glasses, which allow the flow of lubricating oil to be monitored<ref name= Fowler/>Template:Rp
a blower valve, which regulates the steam supplied to the blower (11)<ref name= Fowler/>Template:Rp
a whistle lever, which varies the steam supplied to the whistle (3)<ref name= Fowler/>Template:Rp
blowdown (or blow-off) cocks, which allow water to be ejected from the boiler to avoid concentration of impurities remaining after evaporation of steam.<ref name= Fowler/>Template:Rp
Whistle Steam powered whistle, located on top of the boiler and used for signalling (by the number and length of notes) and warning of approach.<ref name=railwaytech /><ref name="Kirkman p.14">Template:Cite book</ref><ref name=Forney>Template:Cite book</ref>Template:Rp<ref name= Fowler/>Template:Rp
Reach rod Rod linking the reversing lever (US: Johnson Bar) or reversing lever or wheel (UK+) in the cab to the valve gear.<ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
Dynamo / Turbo-generator / Generator Electrical generator driven by a small steam turbine, for the headlight and other locomotive lighting.<ref name= Fowler/>Template:Rp
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.<ref name=railwaytech /><ref name="Kirkman p.14"/><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
Throttle leverTemplate:Font RegulatorTemplate:Font Sets the opening of the throttle valve / regulator valve, (31) which controls the amount of steam entering the cylinders,<ref name=Forney/>Template:Rp<ref name=Kirkman/>Template:Rp<ref name= Fowler/>Template:Rp 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, a vacuum valve (snifting valve)<ref name= Fowler/>Template:Rp 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.
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.<ref name=railwaytech /><ref name="Kirkman p.14"/><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
Air pump / Air compressor Westinghouse pumpTemplate:Font Powered by steam, it compresses air for operating the train air brake system.<ref name= Fowler/>Template:Rp The Westinghouse air brake system is used world-wide;<ref name= Fowler/>Template:Rp 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.<ref name=Forney/>Template:RpVacuum brakes, used historically, do not employ compressors;<ref name= Fowler/>Template:Rp as a result of their relative inefficiency they are no longer in large-scale use.
Smokebox Receives the hot gases that have passed from the firebox through the boiler tubes and, when the throttle/regulator is open, directs them and steam exhausting from the cylinders up the smokestack/chimney, sucking air through the firebed. The smokebox may contain a cinder guard to prevent hot cinders being expelled.<ref name=railwaytech /><ref name=Carlson> Free access subject to limited trial, subscription normally requiredregistration: there are other constraints (such as a cap on daily views) to freely access this source. {{#invoke:citation/CS1|citation
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Blower Vertical pipe below the chimney petticoat pipe, with holes to blow steam upwards. Provides a draught to maintain adequate combustion – and to prevent smoke and flames from entering the cab through the firebox door – when the blastpipe is insufficiently effective, for example when a locomotive is stationary or the throttle/regulator is closed such as when coasting into a station;<ref name=railwaytech /><ref name=Carlson/><ref name= Fowler/>Template:Rp also helps to draw the fire when lighting up.
Petticoat pipe / ApronTemplate:Font Vertical pipe with a bellmouth-shaped lower end extending down from the smokestack into the smokebox; enhances and equalizes draft through the boiler tubes.<ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
Steam pipe Carries steam to the cylinders.<ref name=railwaytech /><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
Smokebox door Hinged circular door to allow service access to the smoke box to fix air leaks and remove cinders.<ref name=railwaytech /><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
Foot board / Run board / Running board / Tread plate Walkway around the locomotive, from the cab front, to facilitate inspection and maintenance.<ref name="Kirkman p.14"/><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
Frame The strong, rigid structure that carries the boiler, cab and engine unit; supported on driving wheels (43) and leading and trailing trucks (14, 46).<ref name= Fowler/>Template:Rp The axles run in slots in the frames.<ref name="Kirkman p.14"/><ref name=Kirkman/>Template:Rp 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<ref name=railwaytech /> but some end-of-era designs included cast steel sub-frames.<ref> BR drawings held by the National Railway Museum, including Template:Small and Template:Small</ref>
Brake shoe / Brake block Cast iron or composite material that rubs on all the driving wheel treads for braking.<ref name=lindahall /><ref name=railwaytech /><ref name= Fowler/>Template:Rp
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.<ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
Valve gear MotionTemplate:Font System of rods and linkages synchronising the valves with the pistons and controls the running direction and power of the locomotive.<ref name=railwaytech /><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
Connecting rod / Main rodTemplate:Anchor 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.<ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
Piston rod Connects the piston to the cross-head.<ref name=railwaytech /><ref name= Fowler/>Template:Rp
Piston 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.<ref name=lindahall /><ref name= Fowler/>Template:Rp
Cylinder Chamber that receives steam from the steam pipe.<ref name=railwaytech /><ref name= Fowler/>Template:Rp
Valve 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.<ref name=railwaytech /><ref name= Fowler/>Template:Rp
Firebox 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.<ref name=lindahall /><ref name=railwaytech /><ref name=Carlson/><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
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.<ref name=railwaytech /><ref name=Carlson /><ref name= Fowler/>Template:Rp
Boiler 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.<ref name=lindahall /><ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
Superheater tubes Pass steam back through the boiler to dry and superheat it for greater efficiency.<ref name=railwaytech /> See Superheater (32).
Throttle valveTemplate:Font Regulator valveTemplate:Font 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).<ref name=lindahall /><ref name=railwaytech /><ref name=Kirkman/>Template:Rp For the other method, see Throttle lever (8).
Superheater Provides additional heat – as much as Template:Convert hotter – to steam that has been generated in the boiler by sending it back through superheat tubes located in the boiler tubes (28), thus increasing engine efficiency and power.<ref name=railwaytech /><ref name=Carlson /><ref name= Fowler/>Template:Rp See also Superheater tubes (30).
SmokestackTemplate:Font Chimney, FunnelTemplate:Font Vertical pipe on top of and inside the smokebox that ejects the exhaust (smoke and steam) above the locomotive.<ref name=lindahall /><ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
Headlight Light on the front of the smoke box to illuminate track ahead and warn of the approach of the locomotive.<ref name="Kirkman p.14"/><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
Brake hose 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.<ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
Water compartment Tank for water to be used by the boiler to produce steam.<ref name= Fowler/>Template:Rp
Coal bunker 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.<ref name=railwaytech /><ref name= Fowler/>Template:Rp
Grate Supports the burning fuel while allowing the products of combustion – ash and small clinker – to drop through.<ref name=railwaytech /><ref name= Fowler/>Template:Rp
Ashpan hopper Collects the ash from the fire.<ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
Equalising beams / Equalising levers / Equalising bars Part of the locomotive suspension system. Its function is to prevent inequalities in the track or roadbed putting an excessive load on an axle, which is especially necessary on uneven or poorly laid tracks. The beam is connected at each end to a spring on an axle or to the end of another equalising beam (usually via a vertical spring hanger) to distribute the locomotive's weight between two or more axles. The frontmost and rearmost components are secured to the locomotive frame. An equalising system links not only the driving-wheel axles but also the trailing and/or leading truck axle(s).<ref name= Fowler/>Template:Rp<ref name="Kirkman p.14"/><ref name=Forney/>Template:Rp
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.<ref name= Fowler/>Template:Rp
Driving wheels DriversTemplate:Font Coupled wheelsTemplate:Font Wheels coupled to the main/side rods, through which the power developed in the cylinders (24) is transformed into tractive power at the rails.<ref name=lindahall /><ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp The weight of bearings and coupling rods on the driving wheels is counterbalanced with cast-in weights to reduce "hammering" on the track when the locomotive is under way.<ref name= Fowler/>Template:Rp
Pedestal / Saddle Connects a leaf spring to a journal box / axle box (40) on a wheel.<ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
Blast pipe Exhaust pipeTemplate:Font Directs exhaust steam up the smokestack/chimney (33), creating a draught that draws hot gases through the firebox (27) and along the boiler tubes (28).<ref name=railwaytech /><ref name= Fowler/>Template:Rp
Pilot truck, Pony truckTemplate:Font Leading bogie / Lead truck / Bissel truckTemplate:Font Wheels at the front of the locomotive<ref name= Fowler/>Template:Rp to guide the front driving wheels around curves, and minimise yawing at higher speeds with the attendant risk of derailment. The truck has some side motion and 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.
Pilot / CowcatcherTemplate:Font A shield made from bars, cast steel or sheet steel to prevent an object on the track from going under the locomotive and possibly derailing the train.<ref name= Fowler/>Template:Rp
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