UIC classification of locomotive axle arrangements

The UIC classification of locomotive axle arrangements, also known as German classification, describes the wheel arrangement of locomotives, multiple units and trams. It is set out in the International Union of Railways (UIC) "Leaflet 650 – Standard designation of axle arrangement on locomotives and multiple-unit sets". It is used in much of the world. The United Kingdom uses a related scheme. The United States uses the simplified AAR wheel arrangement for modern locomotives.

It is a more versatile system than Whyte notation, making fewer assumptions about locomotive layout, and counting axles instead of wheels. Some locomotives are impossible to classify using Whyte notation, but UIC classification handles them easily. It is also much more suited to diesel and electric locomotives.

Garratt-type locomotives are indicated by bracketing or placing plus signs between all individual units.

The most common wheel arrangements in modern locomotives are Bo′Bo′ and Co′Co′.

Standard practice in the United Kingdom was to use the Whyte notation for steam locomotives. The Southern Railway used a system modified from the UIC method: Oliver Bulleid, the Chief Mechanical Engineer, numbered his 4-6-2 Battle of Britain class Pacifics 21C XXX, referring to leading axles, trailing axles and powered axles, UIC classification 2′C1′, Whyte 4-6-2.

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