UIC classification of locomotive axle arrangements
The UIC classification of locomotive axle arrangements is a system for describing the wheel arrangement of locomotives, multiple units and trams. It is provided by the International Union of Railways. The United States use a simplified form (AAR wheel arrangement) for modern locomotives. It is a more versatile system than Whyte notation for classifying steam locomotives by wheel arrangement.
The UIC notation counts pairs of wheels (called "wheelsets" or informally "axles").
Designate a number of consecutive driving axles, starting at A for a single axle. C thus indicates three consecutive pairs of driving wheels.
Designate consecutive non-driving axles, starting with 1 for a single axle.
On many electric and diesel-electric locomotives, axles are individually driven by electric traction motors - this is indicated by suffixing the driving wheel letter with a lower-case "o".
Prime sign ′
Indicates that the axles are mounted on a bogie.
Plus sign +
Indicates that the locomotive or multiple unit consists of permanently coupled and mechanically separated individual vehicles.
Can be used to group letters and numbers describing the same bogie. For example, (A1A) indicates a three axle bogie with the outer two axles driven. When brackets are used a prime is not needed to indicate a bogie. Mallet locomotives can be 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.
Garratt-type locomotives are indicated by bracketing or placing plus signs between all individual units.
The designation can also have additional suffixes, denoting other features of the locomotive:
number of cylinders
Freight train purpose . Also used to indicate shunting locomotives