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UIC classification

The UIC classification is a comprehensive system for describing the wheel arrangement of a locomotive. It can describe the configuration of some steam locomotives impossible to classify by Whyte notation. It is also far more versatile and can be usefully applied to diesel and electric locomotives too.

Whereas the Whyte notation counts wheels, the UIC notation counts axles. Driving and non-driving wheels are distiguished the use of upper case letters of the alphabet (starting at A for a single axle) to denote driving wheels. For example a Pacific-class steam locomotive, denoted 4-6-2 in the Whyte notation, would have a UIC designation of 2C1. On most locomotives (except very early steam locomotives) non-driving wheels are mounted on steerable bogies - this is marked using apostrophes in UIC notation.

Articulated locomotives are indicated using brackets. Mallet locomotives are indicated by bracketing the front power unit - for example, the Union Pacific Big Boy - denoted 4-8-8-4 in Whyte notation, is designated (2'D)D2' in UIC notation. Garrett-type locomotives are indicated by bracketing all individual units.

On many electric and diesel-electric locomotives, axles are individual driven by electric motors - this is indicated by suffixing the driving wheel letter with a lower-case 'o'.

The designation can also have additional suffixes: