Locomotive Wheel Arrangements

The wheel arrangement refers to the distribution of wheels underneath a locomotive. A range of different wheel arrangements have been used.

Steam locomotive wheels are typically described by the Whyte notation. The Whyte notation was developed by Frederick Whyte in 1900. Whyte was a mechanical engineer working for the New York Central Railroad. Most steam locomotives had an arrangement of three groups of wheels: leading wheels, driving wheels and trailing wheels. The number of each group of wheels is counted, with the groups separated by a dash. Thus, 4-8-2 notation refers to a wheel arrangement with four leading wheels (two axles), eight driving wheels (four axles) and two trailing wheels (one axle). In cases where there are no leading or trailing wheels, a zero is inserted in the notation. Thus, 4-6-0 notation refers to a wheel arrangement with four leading wheels (two axles), six driving wheels (three axles) and no trailing wheels. The letter T placed after the wheel arrangement typically indicated the locomotive was a tank engine. Larger articulated locomotives may have more than three groups of wheels. Thus, the New South Wales AD60 Class Beyer-Garratt locomotive had a 4-8-4+4-8-4 wheel arrangement, which involved two separate groupings of four leading wheels (two axles), eight driving wheels (four axles) and four trailing wheels (two axles). In the steam era there was a much greater variety of different wheel arrangements than used by contemporary locomotives.

A separate wheel notation system was devised for diesel and electric locomotives, which was more suitable for describing their wheel arrangements than the Whyte notation. This system uses letters to represent the driving axles, and numbers for non-driving or carrying axles. Letters are used in alphabetical order to indicate the number of driving axles; i.e. A=1, B=2, C=3 D=4. When each axle in a group is individually driven this is indicated by the addition of ‘o’ after the letter. A hyphen is used to separate different wheel groups. Thus, Bo-Bo notation refers to a wheel arrangement with two groups of wheels, each with two individually driven axles (four wheels). A1A-A1A notation refers to a wheel arrangement with two groups of wheels, each with a driving axle, a non-driving axle, and another driving axle. Separate groups of wheels that are permanently linked together are indicated by a ‘+’ sign.

The wheel arrangements described here are the classification systems most commonly used to notate the wheel arrangements of locomotives in Australia, and are used throughout this website. Other notations systems are in use in some countries overseas. The most common wheel arrangement in use for locomotives on Australian railways today is Co-Co.