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Steam locomotive nomenclature is a listing of the components found on a Steam Locomotive.

Fig. 1: Steam Locomotive
Fig. 2: Steam Locomotive and Tender

The following components appear on the illustrations shown above.

1. Boiler — A water container that is heated by hot gases passed through long tubes, thereby producing steam. From time to time the tubes need to be cleaned as they accumulate soot. The smoke box cover at the front of the locomotive is arranged so that it can be opened or removed for access for cleaning the tubes. Because of the high steam demands, locomotive boilers typically have very high heating surface to grate area ratios, commonly 75:1 or greater.
2. Firebox — A box-like furnace chamber that is built into the boiler and usually surrounded by water to contain the fire that is used to heat the boiler. Early locomotives used wood for fuel, whereas later locomotives used coal or oil. With each change in fuel, different types of firebox have been built including the Wootten, which is very wide and shallow to burn poor grades of coal, the Belpaire, which has a flat crown sheet and deep legs surrounded by water jackets. This Belpaire type was used extensively in British locomotives and in machines built by the Pennsylvania Railway in America.
3. Smokebox — The drum on the front of the boiler. Within it the exhaust nozzle or blast pipe creates a partial vacuum by the venturi effect, to draw the exhaust gases through the boiler tubes. When the locomotive is not working, steam can be fed to a ring of steam jets (the blower) around the exhaust nozzle (blast pipe) which produces a small draft.
4. Valve chest — A 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.
5. Cylinder — A hollow device that converts steam energy into mechanical energy.
6. Cross head — A sliding bearing which guides and supports the end of the piston rod and provides a connection of the rod to the main rod (small end).
7. Crosshead guides — These guide the cross head and take the thrust produced from the reciprocating to rotary motion conversion. Also known as slide bars.
8. Piston rod — A rod that connects the piston in the cylinder to the cross-head axle.
9. Valve stem — A rod attached to the valve for moving it during engine operation.
10. Link
11. Reverse lever — A lever within the cab that controls the direction of travel and the timing of the valve. Also called a "johnson bar".
12. Reach rod — This rod links the reversing actuator with the valve gear.
13. Tumbling shaft — A connecting rod which is part of the linkage between the reverse lever and the valve gear.
14. Rocker arm — A link which connects the tumbling shaft to the reach rod.
15. Eccentric rod — Part of the valve gear.
16. Frame — The 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 of them. Frame can be plate frames (cut from steel plate) or bar frames (fabricated from steel bars or a one piece steel casting including the cylinders).
17. Equalizing levers or bars This is a lever free to pivot about its center which is firmly affixed to the locomotive frame. The free ends of the lever are connected to the leaf springs associated with each wheel.
18. Springs A built up leaf spring is located typically above, and sometimes below each drive wheel. These are connected at their middle to the journal box of the axle they are centered over. The ends of these springs connect to the locomotive frame either through a hanger, or through an equalizing lever or bar.
19. Pedestal or saddle A link which connects the clamp at the center of the springs to the bearing journal.
20. Injector — Steam driven vacuum pumps used to inject water into the boiler under pressure.
21. Boiler check valve — A check valve at the point where water enters the boiler from the injectors; used to prevent backflow.
22. Steam Dome — An expansion at the top of the boiler to allow space for the heated steam to collect; the throttle valve for the locomotive is located here.
23. Auxiliary Steam Dome — A collection point for steam used to power locomotice appliances such as the generator turbine, air compressor pumps, and steam lubricators. On many locomotives, a steam manifold serves the same purpose and is mounted with in the cab.
24. Sand box — A container for holding sand used to improve traction especially on wet or icy rails.
25. Cab — A compartment from which the crew consisting of an engineer and fireman can control the engine and feed or look after the fire box.
26. Air pump — A pump to provide air pressure for operating the brakes (train air brake system).
27. Foot board (running board outside of North America) — A walkway along the locomotive to facilitate inspection and maintenance.
28. Hand rail — A railing above the foot board used for support while walking along the foot board.
29. Pilot truck¹ (leading bogie outside of North America) — Wheels at the front that helped guide the locomotive. Some locomotives do not have a pilot truck.
30. Trailing truck¹ — Wheels at the rear of the locomotive to help support the weight of the cab and fire box. Some locomotives did not have a trailing truck.
31. Drivers¹ — The wheels that deliver locomotive power to the rails. The drivers are weighted so that the center of gravity of the drivers and rods coincides with the center of rotation.
32. Main rod — A steel arm that converts the horizontal motion of the piston into a rotation motion of the Driver. Also called the connecting rod.
33. Side rods. Also known as the coupling rods, especially outside North America.
34. Tender — A container that holds both water for the boiler and combustible fuel such as wood, coal or oil for the fire box.

Notes:

1. Locomotives are classified by the number of wheels on the pilot truck, followed by the number of driving wheels, followed by the number of wheels on the trailing truck. Thus the locomotive pictured above would be classified as a 4-6-2 locomotive. This system is known as Whyte notation.

Additional parts or components of a locomotive:

1. Air hose — A hose for supplying air pressure to release the brakes on the rail cars. A hose is located next to the front and rear couplers.
2. Backhead — The firebox wall in the locomotive cab used for mounting the locomotive controls.
3. Bell — A bell is located on top of the boiler and is used for signaling.
4. Coupler — A device at the front and rear of the locomotive for connecting locomotives and rail cars together.
5. Pilot (also known as the cow catcher) — A protective guard at the front of the locomotive to push aside obstructions.
6. Feedwater heater — An optional routing of the feedwater piping below the smoke box to heat the water before it enters the boiler.
7. Fire box door — A door to the fire box is located in the cab to allow access for the tossing of wood or the shoveling of coal and for lighting of the fire.
8. Gauges — A steam pressure gauge and a water level gauge are located in the cab.
9. Headlight — A lamp at the top of and in front of the smoke box to provide visibility.
10. Smokestack — A short chimney on top of the smoke box to carry away the smoke. Usually extended down inside the smoke box.
11. Whistle — A steam whistle is located on top of the boiler and is used for signaling and as a warning device.
12. Valve gear — A system of rods and other linkages synchronizes the valves with the pistons and controls the direction and power output of the locomotive. The illustrated engine has inside connected Stephenson valve gear, although most parts are not visible in the illustrations.
13. Ejector — A device working on the venturi principle to create vacuum on vacuum braked trains.
14. Vacuum Pump — A pump, often worked via a connection to a cross-head, to maintain vacuum on vacuum braked trains. Extensively used by the GWR in the UK.
15. Vacuum hose — A hose for supplying vacuum to release the brakes on the carriages and freight vehicles. A vacuum hose is usually located adjacent to the front and rear couplers. N.b. dual fitted locomotives would have vacuum and air hoses to allow connection of the continuous braking system to both types of rolling stock.de:Dampflokomotive (Bauart)

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