Wheel arrangement

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In rail transport, a wheel arrangement or wheel configuration is a system of classifying the way in which wheels are distributed under a locomotive.^[1] 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.

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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 instead 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 shunter locomotives (US: switcher locomotives) in the UK.

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Geared steam locomotives such as Shays, Heislers, and Climaxes do not have a standard wheel arrangement classification system. Instead of being classified by wheel arrangement, they are instead classified by their design and their number of trucks.

No. of trucks	American name	Picture scheme
2-truck	Class A Shay, Class B Shay, Class A Climax, Class B Climax, Heisler	oo oo
3-truck	Class C Shay, Class C Climax, Heisler	oo oo oo
4-truck	Class D Shay	oo oo oo oo

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• wheel arrangement (P2978) (see uses)

• Bo-Bo
• Bo-Bo-Bo
• Co-Co locomotives