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The UIC classification is a comprehensive system for describing the wheel arrangement of a locomotive. It is the system generally used in much of the world, but it was not historically used in the United Kingdom or United States; a simplified form is used in both countries for modern locomotives. It is a more versatile system, making fewer assumptions about locomotive layout than does the Whyte notation. In the latter notation, some locomotives are effectively impossible to classify; UIC classification handles them easily. The UIC classification is also much more suited to diesel and electric locomotives.
Whereas the Whyte notation counts wheels, the UIC notation counts axles. Driving and non-driving axles are distiguished the use of upper case letters of the alphabet (starting at A for a single axle) to denote driving axles. 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 axles are mounted on steerable bogies - this is marked using apostrophes in UIC notation: 2'C1'.
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 individually driven by electric motors - this is indicated by suffixing the driving wheel letter with a lower-case 'o' or a '0' (zero).
The designation can also have additional suffixes, denoting other features of the locomotive:
- h: Superheated Steam (German: Heißdampf)
- n: Saturated Steam (German: Naßdampf)
- v: Compound (German: Verbund)
- Turb: Turbine
- t: Tank locomotive
and sometimes followed by a number, indicating the number of cylinders in a steam locomotive.
The most common wheel arrangements in modern locomotives are Bo'Bo' or Co'Co', whilst steam locomotives and other older locomotives can have many different wheel arrangements.
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