Wheel arrangement

In rail transport, a wheel arrangement or wheel configuration is a system of classifying the way in which wheels are distributed under a locomotive. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and connections, with the adopted notations varying by country. Within a given country, different notations may also be employed for different kinds of locomotives, such as steam, electric, and diesel powered.

Especially in steam days, wheel arrangement was an important attribute of a locomotive because there were many different types of layout adopted, each wheel being optimised for a different use (often with only some being actually "driven"). Modern diesel and electric locomotives are much more uniform, usually with all axles driven.

Major notation schemes

The main notations are the Whyte notation (based on counting the wheels), the AAR wheel arrangement notation (based on counting either the axles or the bogies), and the UIC classification of locomotive axle arrangements (based on counting either the axles or the bogies).

Whyte notation is generally used for steam locomotives throughout the United States, Canada, the United Kingdom and Ireland. For diesels and electrics, North America uses the AAR wheel arrangement scheme while British practice uses a slightly simplified form of the European UIC classification scheme (except for small diesel shunters, where Whyte notation is used).

In mainland Europe, the UIC classification scheme is generally used for all locomotive types including steam, with some exceptions. In France, the UIC classification is used for diesels and electrics while a scheme similar to the Whyte notation, but counting axles insteads of wheels, is used for steam locomotives. Notably, Switzerland had its own separate notation system until 1989, with the Swiss locomotive and railcar classification now only retained for its narrow gauge railways.

• AAR wheel arrangement - Used largely throughout the US and Canada for diesel and electric locomotives.
• UIC classification - Used in mainland Europe for all locomotive types. Used in the UK for electric and large diesel locomotives.
• Whyte notation - Used in North America, the UK and Ireland for steam locomotives, and for shunters (US: switchers) in the UK.

Comparison from wheel arrangements and wheel picture

VDEV/VMEV/UIC-System	Whyte-Notation	American name	picture scheme
A1	0-2-2		Oo
1A	2-2-0	Planet	oO
1A1	2-2-2	Patentee	oOo
2’A	4-2-0	Crampton, Norris	ooO
3A	6-2-0	Crampton	oooO
B	0-4-0	Four-Wheel-Switcher	OO
1’B	2-4-0	Hanscom	oOO
1'B1'	2-4-2	Columbia	oOOo
2’B	4-4-0	American, Eight-Wheeler	ooOO
2’B1’	4-4-2	Atlantic	ooOOo
2’B2’	4-4-4	Jubilee	ooOOoo
C	0-6-0	Six-Wheel-Switcher	OOO
1’C	2-6-0	Mogul	oOOO
2’C	4-6-0	Ten-Wheeler	ooOOO
1’C1’	2-6-2	Prairie	oOOOo
2’C1’	4-6-2	Pacific	ooOOOo
1’C2’	2-6-4	Adriatic	oOOOoo
2’C2’	4-6-4	Hudson, Baltic	ooOOOoo
D	0-8-0	Eight-Wheel-Switcher	OOOO
1’D	2-8-0	Consolidation	oOOOO
1’D1’	2-8-2	Mikado, Mac Arthur	oOOOOo
1’D2’	2-8-4	Berkshire	oOOOOoo
2’D	4-8-0	Twelve-Wheeler, Mastodon	ooOOOO
2’D1’	4-8-2	Mountain, Mohawk (NYC)	ooOOOOo
2’D2’	4-8-4	General Service (SP), Golden State (SP), Northern, Niagara (NYC), Wyoming	ooOOOOoo
E	0-10-0	Ten-Wheel Switcher	OOOOO
E1’	0-10-2	Union	OOOOOo
1’E	2-10-0	Decapod	oOOOOO
2'E	4-10-0	Mastodon	ooOOOOO
1’E1’	2-10-2	Santa Fe	oOOOOOo
1’E2’	2-10-4	Texas	oOOOOOoo
2’E1’	4-10-2	Texas, Southern Pacific, Overland	ooOOOOOo
1’F	2-12-0	Centipede	oOOOOOO
1’F1’	2-12-2	Javanic	oOOOOOOo
2’F1’	4-12-2	Union Pacific	ooOOOOOOo
C’C	0-6-6-0	Erie (Mallet-Lok)	OOO OOO
(1’C)C	2-6-6-0	nameless (Mallet-Lok)	oOOO OOO
(1’C)C1’	2-6-6-2	Mallet Mogul (SP), Prairie Mallet (ATSF)	oOOO OOOo
(1’C)C2’	2-6-6-4	nameless (Mallet-Lok)	oOOO OOOoo
(2’C)C2’	4-6-6-4	Challenger (Mallet-Lok)	ooOOO OOOoo
(1’C)C3’	2-6-6-6	Allegheny (Mallet-Lok)	oOOO OOOooo
D’D	0-8-8-0	Angus (Mallet-Lok)	OOOO OOOO
(1’D)D	2-8-8-0	Bullmoose (Mallet-Lok)	oOOOO OOOO
(1’D)D1’	2-8-8-2	Chesapeake, Mallet consolidation (Mallet-Lok)	oOOOO OOOOo
(1’D)D2’	2-8-8-4	Yellowstone (Mallet-Lok)	oOOOO OOOOoo
(2’D)D1’	4-8-8-2	Articulated consolidation (Mallet-Lok)	ooOOOO OOOOo
(2’D)D2’	4-8-8-4	Big Boy (Mallet-Lok)	ooOOOO OOOOoo
(1’E)E1’	2-10-10-2	Virginian (Mallet-Lok)	oOOOOO OOOOOo
(2’C1’)(1’C2’)	4-6-2 + 2-6-4	Double Pacific (Garratt-Lok)	ooOOOo oOOOoo
(2’D1’)(1’D2’)	4-8-2 + 2-8-4	Double Mountain (Garratt-Lok)	ooOOOOo oOOOOoo

See also

• Co-Co locomotives