A postcard photo of the Baltimore and Ohio train The Cincinnatian when it was a streamlined steam locomotive, July 1956.

The locomotive shown is No. 5301, "The President Adams", a 4-6-2 Pacific style locomotive.

(Image: Audio-Visual Designs, New Providence, NJ. Photographer: Don Wood, Public domain, via Wikimedia Commons)



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 fueled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the locomotive's boiler to the point where it becomes gaseous and its volume increases 1,700 times. Functionally, it is a steam engine on wheels.

In most locomotives, the steam is admitted alternately to each end of its cylinders in which pistons are mechanically connected to the locomotive's main wheels. Fuel and water supplies are usually carried with the locomotive, either on the locomotive itself or in a tender coupled to it. Variations in this general design include electrically-powered boilers, turbines in place of pistons, and using steam generated externally.

Whyte Steam Locomotive Classification System

Whyte notation is a classification method for steam locomotives, and some internal combustion locomotives and electric locomotives, by wheel arrangement. It was devised by Frederick Methvan Whyte, and came into use in the early twentieth century following a December 1900 editorial in American Engineer and Railroad Journal.

The notation was adopted and remains in use in North America and the United Kingdom to describe the wheel arrangements of steam locomotives, but for modern locomotives, multiple units and trams it has been supplanted by the AAR system in North America.

Common wheel arrangements

The most common wheel arrangements are listed below. The front of the locomotive is to the left.

Whyte classification / Name(s) / No. of units produced

A. Non-articulated locomotives
0-2-2 Northumbrian
2-2-0 Planet
2-2-2 Patentee, Single, Jenny Lind
2-2-4 Aerolite
4-2-0 Jervis
4-2-2 Bicycle, Iron Duke, Single
4-2-4 Huntington
6-2-0 Crampton
0-4-0 Four-coupled
0-4-0+4 Four-coupled as used on railmotors
0-4-2 Olomana
0-4-4 Forney
2-4-0 Porter, 'Old English'
2-4-2 Columbia
2-4-4 Boston
4-4-0 American, eight-wheeler
4-4-2 Atlantic
4-4-4 Reading, Jubilee (Canada)
0-6-0 Six-coupled, Bourbonnais (France)
0-6-2 Branchliner, Webb
0-6-4 Forney six-coupled

0-6-6-0 Fairlie
2-6-0 Mogul 11,000
2-6-2 Prairie
2-6-4 Adriatic
2-6-6 Suburban2
4-6-0 Ten-wheeler
4-6-2 Pacific 6,800
4-6-4 Hudson, Baltic
4-6-6 Used on the Boston and Albany Railroad.
0-8-0 Eight-coupled
0-8-2 River Irt
0-8-4 London
2-8-0 Consolidation 35,000
2-8-2 Mikado, Mike, MacArthur
2-8-4 Berkshire, Kanawha
2-8-6 Used only on four Mason Bogie locomotives
4-8-0 Twelve Wheeler
4-8-2 Mountain, Mohawk (NYC)
4-8-4 Northern, Niagara, Confederation, Dixie, Greenbrier, Pocono, Potomac, Heavy Mountain (Atchison, Topeka, and Santa Fe), Golden State (Southern Pacific), Western, Laurentian (Delaware & Hudson Railroad), General, Wyoming (Lehigh Valley), Governor, Big Apple, GS Series "Daylight" (Southern Pacific)
4-8-6 Proposed by Lima, never built
6-8-6 Turbine, only used on the PRR S2 Steam Turbine 1
0-10-0 Ten-coupled, (rarely) Decapod
0-10-2 Union
2-10-0 Decapod, Russian Decapod
2-10-2 Santa Fe, Central, Decapod (only on the Southern Pacific)
2-10-4 Texas, Colorado (CB&Q), Selkirk (Canada)
4-10-0 Mastodon
4-10-2 Southern Pacific, Overland
0-12-0 Twelve-coupled
2-12-0 Centipede (Germany/France)
2-12-2 Javanic 30
2-12-4 Bulgaria 20
2-12-6 Proposed by Lima, never built
4-12-2 Union Pacific 88
4-14-4 AA20 1 (USSR)

B. Duplex locomotives
0-2-2-0 Used on the Mount Washington Cog Railway
4-4-4-4 (PRR T1) 53
6-4-4-6 (PRR S1) 1
4-4-6-4 (PRR Q2) 26
4-6-4-4 (PRR Q1) 1

C. Articulated locomotives (simple and compound)
2-4-4-2 Skookum
4-4-6-2 AT&SF
0-6-6-0 Erie
2-6-6-0 Mogul Mallet
2-6-6-2 Chesapeake 1,300 (also SP Cab Forward)
2-6-6-4 Adriatic Mallet 60
2-6-6-6 Allegheny, Blue Ridge 68
4-6-6-2 (Southern Pacific class AM-2)
4-6-6-4 Challenger 252
2-6-8-0 (Southern Railway, Great Northern Railway) 39
0-8-8-0 Angus
2-8-8-0 Bull Moose

2-8-8-2 Mikado Mallet 222

2-8-8-4 SP Class AC-9

2-8-8-4 Yellowstone 78

4-8-8-2 Mountain Mallet 195

4-8-8-4 Big Boy 25

2-10-10-2 Mallet (Santa Fe and Virginian railroads) 20

D. Triplex Class Locomotives
2-8-8-8-2 Triplex (Erie RR) 3
2-8-8-8-4 Triplex (Virginian RR) 1

Cab Forward Locomotives

Miscellaneous Components

Franklin Booster

Feedwater Heater


USRA Steam Locomotives

The USRA standard locomotives and railroad cars were designed by the United States Railroad Administration, the nationalized rail system of the United States during World War I.

USRA 0-6-0

USRA 0-8-0

USRA Light 2-8-2 "Mikado"

USRA Heavy 2-8-2 "Mikado"

USRA Light 2-10-2 "Santa Fe"

USRA Heavy 2-10-2 "Santa Fe"

USRA Light 4-6-2 "Pacific"

USRA Heavy 4-6-2 "Pacific"

USRA Light 4-8-2 "Mountain"

USRA Heavy 4-8-2 "Mountain"

USRA 2-6-6-2 Mallet

USRA 2-8-8-2