Whyte Notation
The Whyte notation for classifying steam locomotives by wheel arrangement 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 counts the number of leading wheels, then the number of driving wheels, and finally the number of trailing wheels, groups of numbers being separated by dashes. Other classification schemes, like UIC classification and the French, Turkish and Swiss systems for steam locomotives, count axles rather than wheels.

In the notation a locomotive with two leading axles (four wheels) in front, then three driving axles (six wheels) and then one trailing axle (two wheels) is classified as 4-6-2.

Method
Articulated locomotives
Articulated locomotives such as Garratts, which are effectively two locomotives joined by a common boiler, have a + between the arrangements of each engine. Thus a "double Pacific" type Garratt is a 4-6-2+2-6-4. For Garratt locomotives the + sign is used even when there are no intermediate non-powered wheels, e.g. the LMS Garratt 2-6-0+0-6-2. This is because the two engine units are more than just power bogies. They are complete engines, carrying fuel and water tanks. The + sign represents the bridge (carrying the boiler) that links the two engines.

Simpler articulated types such as Mallets, have a jointed frame under a common boiler where there are no non-powered wheels between the sets of powered wheels. Typically, the forward frame is free to swing, whereas the rear frame is rigid with the boiler. Thus a Union Pacific Big Boy is a 4-8-8-4; four leading wheels, one group of eight driving wheels, another group of eight driving wheels, and then four trailing wheels.

Duplex Locomotives
This numbering system is shared by Duplex Locomotives, which have powered wheel sets sharing a rigid frame.

Suffixes
No suffix means a tender locomotive.

T indicates a tank locomotive: in European practice, this is sometimes extended to indicate the type of tank locomotive: T means side tank, PT pannier tank, ST saddle tank, WT well tank. T+T means a tank locomotive that also has a tender.

In Europe, the suffix R can signify rack (0-6-0RT) or reversible (0-6-0TR), the latter being Bi-cabine locomotives used in France.

The suffix F indicates a fireless locomotive (0-4-0F). This locomotive has no tender.

Other suffixes have been used, including ng for narrow-gauge (less than 1,435 mm (4 ft 8 1⁄2 in) ) and CA or ca for compressed air (running on compressed air from a tank instead of steam from a boiler).

Internal combustion locomotives
In Britain, small diesel and petrol locomotives are usually classified in the same way as steam locomotives, e.g. 0-4-0, 0-6-0, 0-8-0. This may be followed by D for diesel or P for petrol, and another letter describing the transmission: E for electric, H hydraulic, M mechanical. Thus 0-6-0DE denotes a six-wheel diesel locomotive with electric transmission. Where the axles are coupled by chains or shafts (rather than side rods) or are individually driven, the terms 4w, 6w or 8w are generally used. Thus 4wPE indicates a four-wheel petrol locomotive with electric transmission. For large diesel locomotives the UIC classification is used.

Limitations
The main limitation of Whyte Notation is that it does not cover non-standard types such as Shay locomotives, which use geared trucks rather than driving wheels. The most commonly used system in Europe outside the United Kingdom is UIC classification, based on German practice, which can define the exact layout of a locomotive.

Naming
In American (and to a lesser extent British) practice, most wheel arrangements in common use were given names, sometimes from the name of the first such locomotive built. For example the 2-2-0 type arrangement is named Planet, after the 1830 locomotive on which it was first used. (This naming convention is similar to the naming of warship classes.)

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

Arrangement
Whyte classification Name  (locomotive front is to the left)

Non-articulated locomotives
Oo  0-2-2 Northumbrian
oO  2-2-0 Planet
oOo  2-2-2 Single, Jenny Lind
oOoo  2-2-4
ooO  4-2-0 Jervis
ooOo  4-2-2 Bicycle
ooOoo  4-2-4
oooO  6-2-0 Crampton
OO  0-4-0 Four-Coupled
OOo  0-4-2
OOoo  0-4-4 Forney
oOO  2-4-0 Porter, 'Old English'
oOOo  2-4-2 Columbia
oOOoo  2-4-4
ooOO  4-4-0 American,Eight-wheeler
ooOOo  4-4-2 Atlantic
ooOOoo  4-4-4 Reading, Jubilee (Canada)
OOO  0-6-0 Six-Coupled, Bourbonnais (France), USRA 0-6-0 (United States)
OOOo  0-6-2
OOOoo  0-6-4 Forney six-coupled
oOOO  2-6-0 Mogul
oOOOo  2-6-2 Prairie
oOOOoo  2-6-4 Adriatic
oOOOooo  2-6-6
ooOOO  4-6-0 Ten-Wheeler (not Britain)
ooOOOo  4-6-2 Pacific
ooOOOoo  4-6-4 Hudson, Baltic
OOOO  0-8-0 Eight-Coupled, USRA 0-8-0 (United States)
OOOOo 0-8-2 
OOOOoo  0-8-4
oOOOO  2-8-0 Consolidation
oOOOOo  2-8-2 Mikado, Mike, MacArthur
oOOOOoo  2-8-4 Berkshire, Kanawha
oOOOOooo  2-8-6 Used only on four Mason Bogie locomotives
ooOOOO  4-8-0 Twelve-Wheeler
ooOOOOo  4-8-2 Mountain, Mohawk
ooOOOOoo  4-8-4 Northern, Niagara, Confederation, Dixie, Greenbrier, Pocono, Potomac, Golden State (Southern Pacific), Western, Laurentian (Delaware & Hudson Railroad), General, Wyoming (Lehigh Valley), Governor, Big Apple, GS Series "Daylight" (Southern Pacific)
ooOOOOooo  4-8-6 Proposed by Lima, never built
oooOOOOooo  6-8-6 (PRR S2 steam turbine locomotive)
ooooOOOOoooo  8-8-8 (Breitspurbahn)
OOOOO  0-10-0 Ten-Coupled, (rarely) Decapod
OOOOOo  0-10-2 Union
oOOOOO  2-10-0 Decapod, Russian Decapod
oOOOOOo  2-10-2 Santa Fe, Central, Decapod (only on the Southern Pacific)
oOOOOOoo  2-10-4 Texas, Colorado (CB&Q), Selkirk (Canada)
ooOOOOO  4-10-0 Mastodon, Gobernador (in honor of El Gobernador)
ooOOOOOo  4-10-2 Reid Tenwheeler, Southern Pacific, Overland
OOOOOO  0-12-0 Twelve-Coupled
oOOOOOO  2-12-0 Centipede
oOOOOOOo  2-12-2 Javanic
oOOOOOOoo  2-12-4
ooOOOOOOo  4-12-2 Union Pacific
ooOOOOOOOoo  4-14-4 AA20

Duplex locomotives
oo OO OO oo  4-4-4-4 (PRR T1)
ooo OO OO ooo  6-4-4-6 (PRR S1)
oo OO OOO oo  4-4-6-4 (PRR Q2)
oo OOO OO oo  4-6-4-4 (PRR Q1)

Mallet (simple and compound) articulated locomotives
OO OO  0-4-4-0 Bavarian BB II
o OO OO  2-4-4-0
OO OO o  0-4-4-2
o OO OO o  2-4-4-2
OOO OOO  0-6-6-0 Erie
o OOO OOO  2-6-6-0 Denver & Salt Lake
o OOO OOO o  2-6-6-2 C&O/N&W. C&O Class H-2 thru H-5. Alco 1912.
o OOO OOO oo  2-6-6-4 Norfolk & Western
o OOO OOO ooo  2-6-6-6 Allegheny, Blue Ridge
oo OOO OOO o  4-6-6-2 (Southern Pacific class AM-2)
oo OOO OOO oo  4-6-6-4 Challenger
o OOO OOOO  2-6-8-0 (Southern Railway, Great Northern Railway)
OOOO OOOO  0-8-8-0 Angus
o OOOO OOOO  2-8-8-0 Bull Moose
o OOOO OOOO o  2-8-8-2 Chesapeake, Norfolk & Western
o OOOO OOOO oo  2-8-8-4 Yellowstone
oo OOOO OOOO o  4-8-8-2 Southern Pacific cab forward classes AC-4 through AC-12 (except AC-9)
oo OOOO OOOO oo  4-8-8-4 Big Boy
o OOOOO OOOOO o  2-10-10-2 (Santa Fe and Virginian railroads)
o OOOO OOOO OOOO o  2-8-8-8-2 Triplex (Erie RR)
o OOOO OOOO OOOO oo  2-8-8-8-4 Triplex (Virginian RR)

Garratt articulated locomotives
OO + OO  0-4-0+0-4-0
OOO + OOO 0-6-0+0-6-0
oOO + OOo  2-4-0+0-4-2
ooOOOo + oOOoo  4-6-2+2-2-4 Pacific and Bicyle
oOOo + oOOo  2-4-2+2-4-2 Double Columbia
oOOO + OOOo  2-6-0+0-6-2 Double Mogul
oOOOo + oOOOo  2-6-2+2-6-2 Double Prairie
oOOOO + OOOOo  2-8-0+0-8-2 Double Consolidation
oOOOOo + oOOOOo  2-8-2+2-8-2 Double Mikado
ooOOo + oOOoo  4-4-2+2-4-4 Double Atlantic
ooOOO + OOOoo  4-6-0+0-6-4
ooOOOo + oOOOoo  4-6-2+2-6-4 Double Pacific
ooOOOoo + ooOOOoo  4-6-4+4-6-4 Double Baltic, Double Hudson
ooOOOO + OOOOoo  4-8-0+0-8-4 Double Mastodon
ooOOOOo + oOOOOoo  4-8-2+2-8-4 Double Mountain
ooOOOOoo + ooOOOOoo  4-8-4+4-8-4 Double Northern

See also:

AAR Wheel Arrangement

Steam Locomotives

Locomotives

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Whyte notation from a handbook for railroad industry workers published in 1906, and a selection of early 20th century locomotive types according to their Whyte notation and their comparative size.

Pennsylvania Railroad class D6 4-4-0 no. 317, built in 1881.

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