Steam Locomotive Terms & Definitions

Published: August 13, 2026

Steam railroading has its own vocabulary—part shop talk, part type nicknames, part engineering jargon. This page collects the definitions and terms long hosted on American-Rails.com’s steam coverage: wheel and type names, boiler and cab hardware, geared loggers, and a few one-off experiments that still turn up in books and captions.

For the full history, Whyte notation, Super Power, builders, and type/builder directories, start at the steam locomotives hub. For railroad language beyond steam (signals, MOW, operations), see the sitewide glossary and terms.

On this page: A–C · D–H · I–M · P–S · T–W · Component pages · Related reading

Union Pacific Big Boys and Challengers resting in Cheyenne September 1956

Nearing the end of their careers, Union Pacific Big Boys and Challengers rest in Cheyenne on September 6, 1956. Note the workers washing 4-8-8-4 #4024. J.E. Shaw photo. American-Rails.com collection.

A–C

All-weather cab: A steam locomotive cab enclosed at the rear to give the crew protection from harsh weather. Also called a vestibule cab.

Articulated: A steam locomotive with two sets of driving wheels under a single boiler. Common wheel arrangements include 2-8-8-4 and 4-6-6-4. (In rolling stock, “articulated” can also mean adjacent car bodies that share trucks—for example certain passenger sets and double-stack well cars.)

Backhead: The rear of a steam locomotive’s firebox, located in the cab, where many of the locomotive’s controls are mounted.

Cab-forward: An articulated steam locomotive type most strongly associated with the Southern Pacific, with the cab at the front of the engine and the smokestack at the rear. Fuel was oil, piped forward to the firebox. The layout kept crews ahead of the noxious fumes and smoke in the many tunnels and snowsheds of SP’s Sacramento Division through the Sierra.

Camelback: A steam locomotive built through about 1927 with the engineer’s cab astride the middle of the boiler rather than at the rear. A small rear overhang remained for the fireman stoking the wide anthracite-burning firebox.

Climax: A geared steam locomotive built by the Climax Manufacturing Company. A pair of inclined cylinders just behind the smokebox drove a transverse shaft geared to a central longitudinal driveshaft that, in turn, drove all the axles through skew bevel gears.

Compound: A steam locomotive arrangement that uses steam after it has partly expanded in one cylinder (or set of cylinders), then pipes it to another cylinder where it expands further and pushes another piston before exhausting up the stack.

Connecting rod: The large steel arm that transfers motion from the piston (via the crosshead) to the driving wheels—one of the most visually distinctive moving parts of a steam locomotive.

Cylinder cocks: Openings on a steam locomotive’s cylinders that release condensed water that collects when the engine is standing or cooling. Opening the cocks on starting prevents hydraulic lock and cylinder damage.

D–H

Decapod: The 2-10-0 wheel arrangement (ten coupled drivers, two-wheel leading truck, no trailing truck).

Displacement: The volume displaced by a complete stroke of the piston.

Doghouse: Nickname for the small shelter often placed on the tender deck to house the head brakeman.

Duplex-drive: A non-articulated steam locomotive with two sets of cylinders and drive wheels under a single rigid frame (neither a classic articulated nor a Mallet compound). The Pennsylvania Railroad refined the idea in the 1940s; the Baltimore & Ohio had experimented earlier.

Feedwater heater: A device that pre-heats water from the tender as it is fed into the boiler. Warming the feedwater saves fuel and reduces thermal shock. See also the component page: feedwater heater.

Heisler: A geared steam locomotive built by the Heisler Locomotive Works. Two cylinders arranged in a “V” under the boiler drove a central longitudinal shaft geared to the outer axle of each truck; side rods connected the outer and inner axles.

I–M

Injector: A device for adding water to a steam locomotive boiler, using steam to force feedwater in against boiler pressure.

Jawn Henry: Experimental coal-fired steam turbine locomotive built in 1954 by Baldwin-Westinghouse for the Norfolk & Western Railway. The machine weighed about 409 tons, was rated at roughly 4,500 horsepower, produced about 199,000 pounds of tractive effort, and could haul more tonnage on less fuel than conventional steam at lower speeds. It was scrapped in 1957.

MacArthur engine: A wartime redesignation of the Mikado-type (2-8-2) steam locomotive in honor of General Douglas MacArthur. A handful of railroads used the name during World War II.

Mallet: An articulated compound steam locomotive with two sets of cylinders, rods, and drivers under one boiler. The non-swiveling rear engine works at boiler pressure; the swiveling front engine uses exhaust steam from the rear. Developed by Swiss inventor Anatole Mallet (French roughly “mal-LAY,” often pronounced “MAL-ley” in the U.S.). Mallets were substantially larger than earlier rigid-frame power and were especially popular from about 1905 into the mid-1920s; many later U.S. articulateds were simple-expansion rather than true compounds.

Mike: Short for a Mikado (2-8-2) steam locomotive.

P–S

Pop valve: The safety or pressure-release valve on a steam locomotive boiler.

Quarter-locked: Condition when a steam locomotive is stopped so the drive rods align with the pistons in a way that prevents starting on steam alone. The two sides of a conventional two-cylinder engine are “quartered” (cranks one-quarter turn out of phase) to prevent quarter-locking and dead-center stalls.

Radius rod: On a steam locomotive, the rod that transfers motion from the valve-gear mechanism to the valve rod (part of the valve gear train on many Walschaerts-equipped engines and related layouts).

Saddle-tank: A steam locomotive that carries its water supply in a tank straddling the boiler (common on switchers and industrial power that run without a large tender).

Shay: A geared steam locomotive designed by Ephraim Shay and built by Lima Locomotive Works—the most common of the classic geared types. Vertical cylinders on the right of a boiler offset to the left drove a longitudinal shaft that drove the axles through bevel gears. Ideal for steep, rough logging and industrial track.

Smoke deflector: Plates or devices placed near the smokestack (often called “elephant ears” when they are large side sheets) that create air currents to lift exhaust clear of the boiler and cab, improving crew vision. See also: elephant ears / smoke deflectors.

Superheater: A series of coils carrying freshly made steam through the flue gases so the steam is heated further (superheated) before it reaches the cylinders. Superheating commonly adds on the order of 25–30 percent more power for the same boiler effort compared with saturated steam.

T–W

Thermic syphon (also siphon): In a steam locomotive firebox, a funnel-shaped steel fabrication connecting the cooler throat-sheet region with the hot crown sheet. Water flows upward through the syphon, improving circulation and more uniform temperatures, and raising fuel efficiency. See also: firebox.

Three-cylinder steam locomotive: A locomotive with a third cylinder under the smokebox between the two outside cylinders. Power reached the drivers through a main rod connected to a specially designed crank axle.

Water glass: A cab gauge that shows boiler water level so the crew can keep the crownsheet safely covered. Low water over the crownsheet is a primary path to boiler explosion.

Related Component & Type Pages

Many terms have fuller feature pages or type profiles on American-Rails.com:

Firebox · Feedwater heater · Smoke deflectors / elephant ears
Walschaerts valve gear · Baker valve gear · Stephenson valve gear
Cowcatcher / pilot · Whyte notation (on the steam hub)
Geared types and famous articulateds appear in the directories on the steam hub

Steam locomotives hub · Locomotive history hub
Diesel locomotives · Electric locomotives
Railroad glossary & terms (sitewide)
Fireman · Engineer history
Tourist trains (where most people still meet steam) · Home

Sources

American-Rails.com steam locomotive definitions (long-running site glossary block on the steam page)
Solomon, Brian. The Majesty of Big Steam; related steam and builder studies
Builder and railroad literature on Mallet compounds, Super Power practice, geared locomotives, and N&W steam-turbine experiments
Component and type profiles linked from American-Rails.com steam coverage

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Steam Locomotive Terms & Definitions

A–C

D–H

I–M

P–S

T–W

Related Component & Type Pages

Related Reading on American-Rails.com

Sources