__NOTOC__ Main components found on a typical

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, Fuel oil, oil or, rarely, Wood fuel, wood) to heat ...

include: File:Diagram_of_steam_locomotive_components_(March_2021_version).tif, center, 800px, The main components of a typical steam locomotive. Click or hover over numbers to see names. () rect 133 15 172 57 Tender rect 538 17 575 57 Cab rect 667 15 708 57

Safety valve A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds ...

rect 715 15 754 55 Reach rod rect 754 15 789 55

Whistle A whistle is a musical instrument which produces sound from a stream of gas, most commonly air. It is a type of Fipple, fipple flute, and may be mouth-operated, or powered by air pressure, steam, or other means. Whistles vary in size from a s ...

rect 797 13 836 57

Dynamo "Dynamo Electric Machine" (end view, partly section, ) A dynamo is an electrical generator that creates direct current using a commutator. Dynamos employed electromagnets for self-starting by using residual magnetic field left in the iron cores ...

rect 876 9 917 57 Sand dome rect 923 9 963 55 Throttle lever / Regulator rect 1009 11 1056 57

Steam dome The steam dome is a vessel fitted to the top of the boiler of a steam engine. It contains the opening to the main steam pipe and its purpose is to allow this opening to be kept well above the water level in the boiler. This arrangement acts as a ...

rect 1152 9 1194 57 Air pump / Air compressor rect 1220 9 1266 55

Smokebox A smokebox is one of the major basic parts of a steam locomotive exhaust system. Smoke and hot gases pass from the firebox through tubes where they pass heat to the surrounding water in the boiler. The smoke then enters the smokebox, and is ...

rect 1277 9 1324 57 Steam pipe rect 1338 11 1383 55 Smokebox door rect 593 409 640 457 Trailing truck / Rear bogie rect 645 409 688 457 Foot board / Run board / Running board / Tread plate rect 695 409 741 457

Frame A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent. Frame and FRAME may also refer to: Physical objects In building construction *Framing (con ...

rect 752 409 797 455 Brake shoe / Brake block rect 849 412 891 455 Sand pipe rect 895 412 935 455 Coupling rods / Side rods rect 993 410 1037 453

Valve gear The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It can also serve as a reversing ...

rect 1041 410 1083 457 Connecting rod / Main rod rect 1102 410 1148 457 Piston rod rect 1150 410 1187 457

Piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder (engine), cylinder a ...

rect 1185 410 1231 457

Cylinder A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...

rect 1281 412 1322 455

Valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or Slurry, slurries) by opening, closing, or partially obstructing various passageways. Valves are technically Pip ...

rect 1320 409 1361 457 Valve chest / Steam chest rect 697 483 743 531 Firebox rect 810 481 858 531 Boiler tubes and flues rect 878 481 924 532

Boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centra ...

rect 930 481 974 532 Superheater tubes rect 1004 481 1052 536 Throttle / regulator valve rect 1096 483 1143 536

Superheater A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, in some steam engines, and in processes such as steam reforming. ...

rect 1229 483 1277 531 Smokestack / Chimney / Funnel rect 1298 481 1350 531

Headlight A headlamp is a lamp attached to the front of a vehicle to illuminate the road ahead. Headlamps are also often called headlights, but in the most precise usage, ''headlamp'' is the term for the device itself and ''headlight'' is the term for t ...

rect 70 860 122 906 Brake hose rect 131 862 179 908 Water compartment rect 368 860 414 908 Coal bunker rect 571 860 617 908 Grate rect 636 860 690 908 Ashpan hopper rect 784 860 834 906 Journal/axle box rect 849 860 889 906 Equalising beams / Equalising levers / Equalising bars rect 897 860 935 906 Leaf springs rect 937 858 982 906

Driving wheels On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons (or turbine, in the case of a steam turbine locomotive). On a conventional, non-articulated locomotive, the driving wheels are all coupled tog ...

rect 1035 862 1082 904 Pedestal / Saddle rect 1118 863 1165 908 Blast pipe rect 1215 860 1263 910 Pilot truck / Lead truck rect 1289 862 1338 906 Pilot / Cowcatcher rect 1348 862 1396 908 Coupler / Coupling desc bottom-left 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.

Details of the components

Tender
Attached rail vehicle that holds both water for the boiler and fuel such as wood, coal, or oil.
Cab
Footplate
Compartment where the engineer (US+) / driver (UK+) and fireman control the locomotive and tend the steam supply and firebox. They achieve that using various devices, most of which are on the rear surface of the firebox, called the "backhead": *a throttle lever or regulator, which controls the amount of steam entering the cylinders *a reversing lever or (US term) Johnson bar, which controls the timing of the admission of steam into the locomotive's cylinders. 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 * steam pressure gauges, which show the pressure of the steam in the boiler * injector valves, which allow steam to force water into the boiler when needed * water gauges, which allow the level of water in the boiler to be monitored * 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 *a blower valve, which regulates the steam supplied to the blower (11) *a
whistle A whistle is a musical instrument which produces sound from a stream of gas, most commonly air. It is a type of Fipple, fipple flute, and may be mouth-operated, or powered by air pressure, steam, or other means. Whistles vary in size from a s ...
lever, which varies the steam supplied to the whistle (3) * blowdown (or blow-off) cocks, which allow water to be ejected from the boiler to avoid concentration of impurities remaining after evaporation of steam.
Whistle A whistle is a musical instrument which produces sound from a stream of gas, most commonly air. It is a type of Fipple, fipple flute, and may be mouth-operated, or powered by air pressure, steam, or other means. Whistles vary in size from a s ...

Steam powered whistle, located on top of the boiler and used for signalling (by the number and length of notes) and warning of approach.
Reach rod
Rod linking the reversing lever (US: Johnson Bar) or reversing lever or wheel (UK+) in the cab to the
valve gear The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It can also serve as a reversing ...
.
Safety valve A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds ...

Pressure relief valve to stop the boiler pressure exceeding the operating limit.
Dynamo / Turbo-generator / Generator

Electrical generator In electricity generation, a generator, also called an ''electric generator'', ''electrical generator'', and ''electromagnetic generator'' is an electromechanical device that converts mechanical energy to electrical energy for use in an extern ...
driven by a small steam turbine, for the headlight and other locomotive lighting.
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.
Throttle lever
Regulator
Sets the opening of the throttle valve / regulator 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, a vacuum valve ( snifting valve) permits air to be drawn through the
superheater A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, in some steam engines, and in processes such as steam reforming. ...
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 The steam dome is a vessel fitted to the top of the boiler of a steam engine. It contains the opening to the main steam pipe and its purpose is to allow this opening to be kept well above the water level in the boiler. This arrangement acts as a ...

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.
Air pump / Air compressor
Westinghouse pump
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 brake The vacuum brake is a brake, braking system employed on trains and introduced in the mid-1860s. A variant, the automatic vacuum brake system, became almost universal in British train equipment and in countries influenced by British practice. Va ...
s, used historically, do not employ compressors; as a result of their relative inefficiency they are no longer in large-scale use.
Smokebox A smokebox is one of the major basic parts of a steam locomotive exhaust system. Smoke and hot gases pass from the firebox through tubes where they pass heat to the surrounding water in the boiler. The smoke then enters the smokebox, and is ...

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. registration: there are other constraints (such as a cap on daily views) to freely access this source. Components in the smokebox are: :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; also helps to draw the fire when lighting up. :Petticoat pipe / Apron
Vertical pipe with a bellmouth-shaped lower end extending down from the smokestack into the smokebox; enhances and equalizes draft through the boiler tubes.
Steam pipe
Carries steam to the cylinders.
Smokebox door
Hinged circular door to allow service access to the smoke box to fix air leaks and remove cinders.
Trailing truck
Rear bogie
Wheels at the rear of the locomotive to help support the rear of the locomotive and improve riding qualities – see also ''Leading wheel'' (46).
Foot board / Run board / Running board / Tread plate
Walkway around the locomotive, from the cab front, to facilitate inspection and maintenance.
Frame A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent. Frame and FRAME may also refer to: Physical objects In building construction *Framing (con ...

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. BR drawings held by the
National Railway Museum The National Railway Museum (NRM) is a museum in York, England, forming part of the Science Museum Group. The museum tells the story of rail transport in Britain and its impact on society. It is the home of the national collection of historical ...
, including and
Brake shoe / Brake block
Cast iron or composite material that rubs on all the driving wheel treads for braking.
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.
Coupling rods / Side rods
Connect the driving wheels (43) together.
Valve gear The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It can also serve as a reversing ...

Motion In physics, motion is when an object changes its position with respect to a reference point in a given time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and frame of reference to an o ...

System of rods and linkages synchronising the valves with the pistons and controls the running direction and power of the locomotive.
Connecting rod / Main rod
Steel arm that converts the reciprocating motion of the piston into a rotary motion of the
driving wheels On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons (or turbine, in the case of a steam turbine locomotive). On a conventional, non-articulated locomotive, the driving wheels are all coupled tog ...
. The connection between piston and main rod is a crosshead, which slides on a horizontal bar behind the cylinder.
Piston rod
Connects the piston to the cross-head.
Piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder (engine), cylinder a ...

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.
Cylinder A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...

Chamber that receives steam from the steam pipe.
Valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or Slurry, slurries) by opening, closing, or partially obstructing various passageways. Valves are technically Pip ...

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 valve A poppet valve (also sometimes called mushroom valve) is a valve typically used to control the timing and quantity of petrol (gas) or vapour flow into or out of an engine, but with many other applications. It consists of a hole or open-ended ch ...
s.
Valve chest / Steam chest
Valve chamber next to the cylinder (24) containing passageways to distribute steam to the cylinders.
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.
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.
Boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centra ...

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 Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...
– produced by heat from the firebox and boiler tubes – to be above the water surface.
Superheater tubes
Pass steam back through the boiler to dry and superheat it for greater efficiency. See ''Superheater'' (32).
Throttle valve
Regulator valve
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).
Superheater A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, in some steam engines, and in processes such as steam reforming. ...

Provides additional heat – as much as 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. See also ''Superheater tubes'' (30).
Smokestack
Chimney A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator, or fireplace from human living areas. Chimneys are typical ...
, Funnel
Vertical pipe on top of and inside the smokebox that ejects the exhaust (smoke and steam) above the locomotive.
Headlight
Light on the front of the smoke box to illuminate track ahead and warn of the approach of the locomotive.
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 The vacuum brake is a brake, braking system employed on trains and introduced in the mid-1860s. A variant, the automatic vacuum brake system, became almost universal in British train equipment and in countries influenced by British practice. Va ...
system.
Water compartment
Tank for water to be used by the boiler to produce steam.
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.
Grate
Supports the burning fuel while allowing the products of combustion – ash and small clinker – to drop through.
Ashpan hopper
Collects the ash from the fire.
Journal box A bogie or railroad truck holds the wheel sets of a rail vehicle. Axlebox An axlebox, also known as a journal box in North America, is the mechanical subassembly on each end of the axles under a railway wagon, coach or locomotive; it contai ...

Axle box
Housing for the bearing on the
axle An axle or axletree is a central shaft for a rotation, rotating wheel and axle, wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In ...
of a
wheel A wheel is a rotating component (typically circular in shape) that is intended to turn on an axle Bearing (mechanical), bearing. The wheel is one of the key components of the wheel and axle which is one of the Simple machine, six simple machin ...
(43).
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).
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 A bogie or railroad truck holds the wheel sets of a rail vehicle. Axlebox An axlebox, also known as a journal box in North America, is the mechanical subassembly on each end of the axles under a railway wagon, coach or locomotive; it contai ...
/ axle box (40) to the frame.
Driving wheels On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons (or turbine, in the case of a steam turbine locomotive). On a conventional, non-articulated locomotive, the driving wheels are all coupled tog ...

Drivers
Coupled wheels
Wheels coupled to the main/side rods, through which the power developed in the cylinders (24) is transformed into tractive power at the rails. 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.
Pedestal / Saddle
Connects a leaf spring to a journal box / axle box (40) on a wheel.
Blast pipe
Exhaust pipe
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).
Pilot truck, Pony truck
Leading bogie / Lead truck / Bissel truck
Wheels at the front of the locomotive 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 / Cowcatcher
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.
Coupler
Coupling
Device at the front and rear of the locomotive for connecting locomotives and
rolling stock The term rolling stock in the rail transport industry refers to railway vehicles, including both powered and unpowered vehicles: for example, locomotives, Railroad car#Freight cars, freight and Passenger railroad car, passenger cars (or coaches) ...
.

References

External links

List of US–UK terminology
– Railway Technical Website {{Steam engine configurations Glossaries of rail transport Steam locomotive components Steam locomotives Steam locomotive technologies Wikipedia glossaries using ordered lists

Details of the components

See also

References

Further reading

External links