Steam locomotive components
A listing of the components found on typical steam locomotive
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

Guide to steam locomotive components (The image is of a composite imaginary locomotive, not all components are present on all locomotives and not all possible components are present and/or labelled in the illustration above).
1 Tender
Tender locomotive
A tender or coal-car is a special rail vehicle hauled by a steam locomotive containing the locomotive's fuel and water. Steam locomotives consume large quantities of water compared to the quantity of fuel, so tenders are necessary to keep the locomotive running over long distances. A locomotive...

— Container holding both water for the boiler and combustible fuel such as wood, coal or oil for the fire box.
2 Cab
Cab (locomotive)
The cab, crew compartment or driver's compartment of a locomotive is the part of the locomotive housing the train driver or engineer, the fireman or driver's assistant , and the controls necessary for the locomotive's operation....

— Compartment from which the engineer and fireman can control the engine and tend the firebox.
3 Whistle
Train whistle
A train whistle or air whistle, , is an audible signaling device on a steam locomotive used to warn that the train is approaching, and to communicate with rail workers....

— Steam powered whistle, located on top of the boiler and used as a signalling and warning device.
4 Reach rod — Rod linking the reversing actuator in the cab (often a 'johnson bar') to the valve gear
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...

5 Safety valve
Safety valve
A safety valve is a valve mechanism for the automatic release of a substance from a boiler, pressure vessel, or other system when the pressure or temperature exceeds preset limits....

— Pressure relief valve to stop the boiler exceeding the operating limit.
6 Generator — small steam turbine directly coupled with electrical generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

 to headlights etc.
7 Sand dome
Sandbox (railways)
A sandbox is a container on most locomotives and self propelled multiple units, or trams, that run on tramways and adhesion railways...

— Holds sand that can be deposited on the rails to improve traction, especially in wet or icy conditions.
8 Throttle Lever/Regulator — Controls the opening of the regulator/throttle valve (#.31) thereby controlling the supply of steam to the cylinders.
9 Steam dome
Steam dome
A Steam dome is a vessel fitted to the top of the boiler of a steam locomotive. 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...

— Collects the steam at the top of the boiler so that it can be fed to the engine via the regulator/throttle valve.
10 Air pump — Provides air pressure for operating the brakes (train air brake system). This is sometimes called a Westinghouse pump or Knorr pump after George Westinghouse
George Westinghouse
George Westinghouse, Jr was an American entrepreneur and engineer who invented the railway air brake and was a pioneer of the electrical industry. Westinghouse was one of Thomas Edison's main rivals in the early implementation of the American electricity system...

 and Georg Knorr
Georg Knorr
Theodor Georg Knorr , was an engineer and entrepreneur on the field of railroad technology and founder of the company Knorr-Bremse...

. Usually a steam powered pump with steam cylinder coupled with air cylinder.
11 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 exhausted to the atmosphere through the chimney .To assist...

— Collects the hot gases that have passed from the firebox and through the boiler tubes. It may contain a cinder guard to prevent hot cinders being exhausted up the chimney. Usually has a blower to help draw the fire when the regulator is closed. Steam exhausting from the cylinders is also directed up to the chimney through the smokebox to draw the fire while the regulator is open.
Blower may refer to:* USS Blower , a submarine of the United States Navy* a ducted centrifugal fan, especially when used in a heating, ventilating, and air-conditioning system* a supercharger on an internal-combustion engine...

a circular pipe below the chimney petticoat pipe, with holes to blow steam upwards. Provides a draught to encourage the fire when stationary and the blastpipe is not effective. This draught also prevent smoke and flame from entering the cab.
12 Main steam pipe — carries steam to the cylinders.
13 Smoke box door — Hinged circular door to allow service access to the smoke box.
14 Hand rail — Support rail for crew when walking along the foot board.
15 Trailing truck/Rear bogie
Trailing wheel
On a steam locomotive, a trailing wheel or trailing axle is generally an unpowered wheel or axle located behind the driving wheels. The axle of the trailing wheels was usually located on a trailing truck...

— Wheels at the rear of the locomotive to help support the weight of the cab and fire box.
16 Foot board/Running board — Walkway along the locomotive to facilitate inspection and maintenance. UK terminology is Footplate
The footplate of a steam locomotive is a large metal plate that rests on top of the frames and is normally covered with wooden floorboards. It is usually the full width of the locomotive and extends from the front of the cab to the rear of cab or coal bunker just above the buffer beam. The...

17 Frame — Steel beams around which the locomotive is built. The wheels run in slots within the frames, and the cab, fire box, boiler and smoke box are mounted on top. American locomotives usually have bar frames (made from steel bar) or cast steel frames, while British locomotives usually have plate frames (made from steel plate).
18 Brake shoe
Brake shoe
A brake shoe is the part of a braking system which carries the brake lining in the drum brakes used on automobiles, or the brake block in train brakes and bicycle brakes.-Automobile drum brake:...

and brake block — Applied directly to all the driving wheels for braking.
19 Sand pipe — Deposits sand directly in front of the driving wheels to aid traction.
20 Side rods/Coupling rod
Coupling rod
right|thumb|connecting rod and coupling rods attached to a small locomotive driving wheelA coupling rod or side rod connects the driving wheels of a locomotive. Steam locomotives in particular usually have them, but some diesel and electric locomotives, especially older ones and shunters, also have...

— Connects the driving wheels together.
21 Valve gear
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...

— System of rods and linkages synchronising the valves with the pistons and controls the direction and power output of the locomotive.
22 Main rod/Connecting rod
Connecting rod
In a reciprocating piston engine, the connecting rod or conrod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts linear motion into rotating motion....

— Steel arm that converts the horizontal motion of the piston into a rotation motion of the driver wheels
Driving wheel
On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...

. The connection between piston and main rod is supported by a horizontal slide-bar behind the cylinder.
23 Piston rod
Piston rod
In a piston engine, a piston rod joins a piston to a connecting rod.Many internal combustion engines, and in particular all current automobile engines, do not have true piston rods, and the term piston rod is often used as a synonym for connecting rod in the context of these engines.All engines...

— Connects the piston to the cross-head axle, which drives the main/connecting rods.
24 Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...

— Driven backward and forward within the cylinder by steam pressure, producing mechanical motion from steam expansion.
25 Valve — Controls the supply of steam to the cylinders, timing is synchronised by the valve gear connect to the Drivers. Steam locomotives may have slide valves
D slide valve
The slide valve is a rectilinear valve used to control the admission of steam into, and emission of exhaust from, the cylinder of a steam engine.-Use:...

, piston valve
Piston valve
A piston valve is a device used to control the motion of a fluid along a tube or pipe by means of the linear motion of a piston within a chamber or cylinder.Examples of piston valves are:...

s or poppet valve
Poppet valve
A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...

26 Valve chest/steam chest — Small chamber (sometimes cylindrical) above or to the side of the main cylinder containing passageways used by the valves to distribute live steam to the cylinders.
27 Firebox — Furnace chamber that is built into the boiler and usually surrounded by water. Almost anything combustible can be used as fuel but generally coal, coke, wood or oil are burnt.
28 Boiler tubes — Carry hot gasses from the fire box through the boiler, heating the surrounding water.
29 Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...

— Water container that is heated by hot gases passed through boiler tubes, thereby producing steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...

30 Superheater tubes — Pass steam back through the boiler to dry out and 'super heat' the steam for greater efficiency.
31 Regulator/Throttle valve — Controls the amount of steam delivered to the cylinders (also see #.8).
32 Superheater
A superheater is a device used to convert saturated steam or wet steam into dry steam used for power generation or processes. There are three types of superheaters namely: radiant, convection, and separately fired...

— Feeds steam back through boiler tubes to superheat (heat beyond just boiling point) the steam to increase the engine efficiency and power.
33 Chimney
Chimney (locomotive)
The chimney of a steam locomotive is a part of the exhaust system which helps in the creation of draught through the boiler and carries the exhaust steam and smoke clear of the driver's line of sight...

— Short chimney on top of the smokebox to carry the exhaust (smoke) away from the engine so that it doesn't obscure the engineer's vision. Usually extended down inside the smokebox - the extension is called a petticoat. Some railways, e.g. the Great Western Railway
Great Western Railway
The Great Western Railway was a British railway company that linked London with the south-west and west of England and most of Wales. It was founded in 1833, received its enabling Act of Parliament in 1835 and ran its first trains in 1838...

, fitted a decorative copper cap to the top of the chimney.
34 Headlight — Lamp on front of the smoke box to provide forward visibility.
35 Brake hose — Air or vacuum hose for transmitting braking control to attached rolling stock. See air brake
Air brake (rail)
An air brake is a conveyance braking system actuated by compressed air. Modern trains rely upon a fail-safe air brake system that is based upon a design patented by George Westinghouse on March 5, 1872. The Westinghouse Air Brake Company was subsequently organized to manufacture and sell...

 and vacuum brake
Vacuum brake
The vacuum brake is a 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 those countries influenced by British practice. Vacuum brakes also enjoyed a brief period of adoption in...

36 Water compartment — Container for water used by the boiler to produce steam that is subsequently usually exhausted from the cylinders.
37 Coal bunker — Fuel supply for the furnace. Variations may hold wood, coke, or oil. Fed to the firebox either manually or, in later engines, mechanically.
38 Grate — Holds the burning fuel and allows unburnable ash to drop through.
39 Ashpan hopper — Collects the unburnable ash from spent fuel.
40 Journal box — Contains the bearing for a driver wheel's
Driving wheel
On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...

An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

41 Equalising levers/Equalising bars — Part of the locomotive suspension system, connected to Leaf Springs, free to pivot about their centre which is firmly fixed to the frame.
42 Leaf Springs
Leaf spring
Originally called laminated or carriage spring, a leaf spring is a simple form of spring, commonly used for the suspension in wheeled vehicles...

— Main suspension element for the locomotive. For each driver wheel there is a leaf spring suspending its axle's journal box.
43 Driver
Driving wheel
On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...

— Wheel driven by the pistons to propel the locomotive. Drivers are balanced by weights so that the centre of gravity, of the drivers and rods, coincides with the center of rotation. There are 3 sets of driving wheels in this example.
44 Pedestal or saddle — Connects a leaf springs to a driver wheel's
Driving wheel
On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...

 journal box.
45 Blast pipe
The blastpipe is part of the exhaust system of a steam locomotive that discharges exhaust steam from the cylinders into the smokebox beneath the chimney in order to increase the draught through the fire.- History :...

— Directs exhaust steam up the chimney, creating a draught that draws air through the fire and along the boiler tubes.
46 Pilot truck/Leading bogie
Leading wheel
The leading wheel or leading axle of a steam locomotive is an unpowered wheel or axle located in front of the driving wheels. The axle or axles of the leading wheels are normally located in a truck...

— Wheels at the front to guide the locomotive along the track.
47 Coupler
Coupling (railway)
A coupling is a mechanism for connecting rolling stock in a train. The design of the coupler is standard, and is almost as important as the railway gauge, since flexibility and convenience are maximised if all rolling stock can be coupled together.The equipment that connects the couplings to the...

— Device at the front and rear of the locomotive for connecting locomotives and rail cars together.
Snifting valve
Snifting valve
A snifting valve is an automatic anti-vacuum valve used in a steam locomotive when coasting. The word Snift imitates the sound made by the valve....

(not shown) — An anti-vacuum valve which permits air to be drawn through the superheater
A superheater is a device used to convert saturated steam or wet steam into dry steam used for power generation or processes. There are three types of superheaters namely: radiant, convection, and separately fired...

 and cylinders which allows the engine to coast freely when the regulator is closed.

Related terminology

  • Tractive effort
    Tractive effort
    As used in mechanical engineering, the term tractive force is the pulling or pushing force exerted by a vehicle on another vehicle or object. The term tractive effort is synonymous with tractive force, and is often used in railway engineering to describe the pulling or pushing capability of a...

  • Power classification
  • Drawbar horsepower
  • Cylinder
    Cylinder (steam locomotive)
    The cylinders of a steam locomotive are the components that convert the power stored in the steam into motion.Cylinders may be arranged in several different ways.-Early locomotives:...

  • Glossary of boiler terminology
    Glossary of boiler terminology
    Boilers for generating steam or hot water have been designed in countless different shapes, sizes and configurations. An extensive terminology has evolved to describe their common features...

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