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Wheel arrangements

Locomotives are often described by their wheel arrangement, which is the configuration of leading, driving and trailing wheels. For British steam locomotives, the most widely adopted standard for this purpose is the Whyte notation, named after the American engineer F W Whyte who first proposed it in 1900. Although strictly a description rather than an object name, wheel arrangements often form part of a locomotive's type designation. It has been suggested that all known wheel arrangements expressed using the Whyte system should be included individually in this thesaurus but, after some debate, the general consensus is that this would add a rather tedious and unhelpful list of over 150 terms (considerably more if all the permutations of tank types are added). Since the Whyte notation allows almost any configuration of wheels to be enumerated by following a standard set of rules, it is felt more helpful to outline the basic formula here.

To derive the Whyte wheel arrangement, the number of wheels is counted starting with leading undriven carrying wheels, followed after a dash by the number of coupled driving wheels, another dash and finally the number of undriven trailing wheels. Thus a six-coupled locomotive (3 axles, 6 wheels linked by connecting rods) with no leading or trailing wheels would be expressed as an 0-6-0 locomotive. A locomotive with a leading four-wheel bogie, four coupled driving wheels and a trailing two-wheeled truck would be described as having a 4-4-2 wheel arrangement.

Where the driving wheels are not coupled, each pair must be shown separately. For example, a locomotive with a leading two-wheeled truck, two separately driven axles and no trailing wheels is described as 2-2-2-0. The same principle is applied to articulated locomotives, each set of coupled wheels being enumerated separately (e.g. 2-6-6-2). Depending on the method of articulation, a plus symbol may be inserted between each distinct articulated 'unit' (e.g. 4-6-2+2-6-4). Further information on applying the Whyte notation to some of the more complex articulated designs built in Britain for export may be provided in future editions of this thesaurus.

Tank engines are indicated by appending a 'T' to the wheel arrangement (e.g. 0-6-2T). This can be further refined by using 'ST' to indicate a saddle tank (e.g. 0-4-0ST), 'PT' for pannier tanks (e.g. 0-6-0PT) and 'WT' for a well tank (e.g. 0-4-2WT). Under this system a 'T' on its own indicates side tanks.

Some of the more common wheel arrangements had names (e.g. 'Atlantic' represents a 4-4-2 configuration). Those popular in Britain are included in the main thesaurus listing.

Whilst the Whyte notation has been applied to diesel shunting locomotives with coupled wheels, most diesel and electric locomotives are described using a notation developed from the European system of giving letters to the driven axles and numerals to the carrying axles. Where the driven axles have separate motors, the driven axle letter is followed by a letter o. Thus a diesel locomotive having two sets of three separately driven axles, flanked by single leading and trailing axles, is designated 1Co-Co1. A locomotive with a pair of six wheel bogies with the outer axles in each bogie motored, is classed as A1A-A1A and so on.

Four and six wheel diesel or petrol shunting locomotives with uncoupled wheels are sometimes designated 4w or 6w.

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