Locomotive Wheel Arrangement Designations

The arrangement of the wheels on a locomotive has always been indicative of the construction and type of services for which the locomotive is appropriate. At the present day, this has boiled down to a few rather boring but serviceable types, but historically, especially in the period of early development, there was great variety, and some kind of shorthand is useful. The conventions given here are those adopted by the Union of German Railway Administrations (VDEV) in 1908 and amended over time. They apply mainly to electrically driven locomotives, but can be used for any kind of power unit. As is common on the Continent, axles are numbered instead of wheels.

Carrying axles are denoted by numbers, and are assumed to be held in a rigid frame: 1, 2, and so on. Carrying axles bear only a load, and are not driven. They are usually smaller than driving axles.

Driving axles are denoted by capital letters, as A, B, C, and so forth for 1, 2, or 3 axles in a rigid frame. The letters alone imply that the axles are connected mechanically so that they rotate as a group, not independently. This was usually done by means of side rods, but can also take place through gears. If the axles are independently driven, and can rotate with respect to each other, they are denoted Bo, Co, and so on. If they are in groups that can rotate with respect to the locomotive body, an apostrophe is added: C' or Bo', for example.

A group of axles such as 1B means a frame in which one carrying axle and two connected driving axles are arranged, in that order. If the carrying axle can move sideways or turn with respect to the frame, an apostrophe is added: 1'B.

If two groups are arranged in separate rigid frames that can turn under the body of the locomotive, they are separated by parentheses. For example, (1'Bo)(Bo1') is a locomotive supported on two frames, each with a leading carrying axle that can move sideways and two independent driving axles.

If two groups are under separate bodies that are permanently connected to make a single unit, a + sign is used instead: 1'Bo+Bo1'.

The most common wheel arrangements at the present time are Bo'Bo' and Co'Co'. The apostrophes are often omitted, although strictly BoBo means that the four axles are in two groups on the same frame (which would be called Do). BB, B'B', and D are all quite distinct and possible, however.

[Bipolar] The locomotive shown is a Milwaukee Road "bipolar" 3000VDC locomotive with Batchelder gearless drive on 12 axles. The wheel arrangement is (1Bo)Do'Do'(Bo1), assuming that the leading and trailing carrying axles are fixed in the same frame with the two driving axles accompanying them. Note that the body is in three jointed pieces, giving the long locomotive considerable flexibility. These were passenger engines for the Cascade Mountain crossing, with a top speed of 60 mph.

The Whyte system for steam locomotives, used in the United States and Britain, is not quite as detailed. This system gives the number of leading carrying wheels, the number of driving wheels, and the number of trailing carrying wheels, separated by dashes. A 4-6-2 is a locomotive with four leading carrying wheels, six driving wheels, and two trailing carrying wheels. Whether the wheels have sideways or pivoting freedom is not explicitly expressed. A 2-6-6-4 is a locomotive with two leading carrying wheels, two sets of six driving wheels, and four trailing wheels. Usually, the leading and trailing wheels can move sideways or pivot, and the two sets of driving wheels are jointed, the rear set with the main frame, and the front set movable from side to side, called articulated. On the other hand, a 4-4-4-4, represented by the Pennsylvania's T1 passenger engine, had a rigid frame but two sets of cylinders, rods, and driving wheels, and was called a duplex. To distinguish articulated from duplex locomotives, a plus sign was used instead of a dash: 2-6+6-4. A T following the designation indicated a tank locomotive (water and fuel carried on the locomotive, not on a separate tender). Sometimes the type of tank was expressed: PT -- pannier tank; WT - well tank; ST - saddle tank.

The VDEV notation can be used for steam locomotives. A 2-8-0 becomes a 1'D, a 2-6-6-4 a (1'C)C2' or 1'C+C2' (if the leading part is taken as a separate unit), and a 4-4-4-4 a 2'BB2'. This was done in Austria, for example, but the apostrophes were omitted.

In the United States, a simplified VDEV notation was used for electric and diesel locomotives, since the Whyte system was not very informative with its 0-4-4-0 and 0-6-6-0 designations, and required extension. Numbers were used for carrying axles, letters for driving axles, and - separating individual sets. A Bo'Bo; becomes simply B-B, a Co'Co' a C-C. Some passenger diesel engines could then be denoted A1A-A1A, as they had trucks with a powered axle at each end, and a carrying axle in the middle, all in the same frame.

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Composed by J. B. Calvert
Last revised 24 August 1999