UIC classification of locomotive axle arrangements

The UIC classification of locomotive axle arrangements, sometimes known as German classification or German system, 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 the Whyte notation. The United States uses the simplified AAR wheel arrangement for modern locomotives.

Structure

 * Upper-case letters : Indicate driving axles, starting at A for a single axle. C thus indicates three consecutive pairs of driving wheels.
 * Lower-case "o" : Related to driving axles (minimum 2, "B"), mean that are individually driven by separate traction motors.
 * Numbers : Consecutive non-driving axles, starting with 1 for a single axle.
 * Prime sign " ′ " : The axles are mounted on a bogie.
 * Brackets : Groups 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, 4-8-8-4 in Whyte notation, is (2′D)D2′ in UIC notation.
 * Plus sign "+" : The locomotive or multiple unit consists of permanently coupled but mechanically separate vehicles.

Garratt-type locomotives are indicated by bracketing or placing plus signs between all individual units.
 * Other suffixes:
 * h: superheated Steam (Heißdampf)
 * n: saturated Steam (Nassdampf)
 * v: compound (Verbund)
 * Turb: turbine
 * number: number of cylinders
 * t: tank locomotive
 * tr: tram (urban) locomotive
 * E: Engerth-type locomotive
 * G: freight (Güterzug). Also used to indicate shunting locomotives
 * P: passenger (Personenzug)
 * S: fast passenger (Schnellzug)

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

Examples

 * (A1A)(A1A)
 * Two bogies or wheel assemblies under the unit. Each bogie has one powered axle, one idle axle, and one more powered axle. All powered axles are individually driven by traction motors.


 * BB
 * Four powered axles all mounted in the locomotive's frame, driven in pairs, i.e. each pair of axles is connected by driving rods or gears. Compare with "D" below.


 * B′B′
 * Two bogies or wheel assemblies under the unit. Each bogie has two powered axles, connected by driving rods or gears.


 * Bo′(A1A)
 * Two bogies or wheel assemblies. The "Bo′" bogie is under one end of the unit, and has two powered axles, while the "(A1A)" bogie under the other end of the unit has one powered axle, one idle axle, and another powered axle. All powered axles are individually driven by traction motors.


 * Bo′Bo′
 * Two bogies or wheel assemblies under the unit. Each bogie has two powered axles individually driven by traction motors. Three-quarters of all modern locomotives (and power cars of self-propelled trains) are configured in either this or the "B′B′" arrangement.


 * Bo′Bo′Bo′
 * Three bogies or wheel assemblies under the unit. Each bogie has two powered axles individually driven by traction motors. See also: Bo-Bo-Bo.


 * C
 * Three powered axles, connected by driving rods or gears, all mounted in the locomotive's frame (Whyte notation: 0-6-0).


 * C′C′
 * Two bogies or wheel assemblies under the unit. Each bogie has three powered axles, connected by driving rods or gears. One such example of this type is Southern Pacific 9010.


 * Co′Co′
 * Two bogies or wheel assemblies under the unit. Each bogie has three powered axles individually driven by traction motors. See also: Co-Co.


 * (2Co)(Co2)
 * A locomotive with two bogies, each with two leading axles and three individually powered axles. A number of Japanese electric locomotives used this wheel arrangement, including the JNR Class EF58, and the PRR GG1.


 * D
 * Four powered axles, connected by driving rods or gears, all mounted in the locomotive's frame (Whyte notation: 0-8-0).


 * 1′D1′
 * One leading idle (non-driven) axle mounted in a bogie, four driven axles mounted in the frame and connected by driving rods or gears, followed one trailing idle axle mounted in a bogie (Whyte notation: 2-8-2).


 * E
 * Five powered axles, mounted in the locomotive's frame (Whyte notation: 0-10-0).


 * 2′D1′h3S
 * Two front leading axles grouped in a bogie, four driving axles, one trailing axle in a bogie, simple steam expansion, superheated steam, three-cylinders, for fast trains.


 * 1′E1′h2Gt
 * One front leading axle in a bogie, five driving axles, one rear trailing axle, simple steam expansion, superheated steam, two-cylinder machine, for freight trains, tank engine.


 * 1′Dn4vP
 * One front leading axle in a bogie, four driving axles, saturated steam, four-cylinder machine, compound (double steam expansion), for passenger trains.


 * D′Dh4vtG
 * No front leading axle, four driving axles in a bogie, four more driven axles mounted in the frame (Mallet locomotive), superheated steam, four-cylinder machine, compound (double steam expansion), tank locomotive, for freight trains.

United Kingdom
Standard practice in the United Kingdom was to use the Whyte notation for steam locomotives. During World War 2, the Southern Railway used a system modified from the UIC method: Oliver Bulleid, the Chief Mechanical Engineer, numbered the first nineteen of his 4-6-2 Merchant Navy class Pacifics 21C1 to 21C19, and the first seventy of his 4-6-2 West Country and Battle of Britain class Light Pacifics 21C101 to 21C170, referring to leading axles, trailing axles and powered axles (cf. UIC classification 2′C1′, Whyte 4-6-2). Also, all forty of Bulleid's "Austerity" 0-6-0 Q1 class were similarly numbered C1 to C40, and his two electric locomotives (later British Rail Class 70) were numbered CC1 and CC2 (UIC classification Co′Co′, Commonwealth Co-Co).