Whyte notation



The Whyte notation for classifying steam and some internal combustion locomotives by wheel arrangement was devised by Frederick Methvan Whyte, and came into use in the early twentieth century following a December 1900 editorial in American Engineer and Railroad Journal. The notation counts the number of leading wheels, then the number of driving wheels, and finally the number of trailing wheels, numbers being separated by dashes. Other classification schemes, like UIC classification and the French, Turkish and Swiss systems for steam locomotives, count axles rather than wheels.

In the notation a locomotive with two leading axles (four wheels) in front, then three driving axles (six wheels) and then one trailing axle (two wheels) is classified as 4-6-2, and is commonly known as a Pacific.

Articulated locomotives
Articulated locomotives such as Garratts, which are effectively two locomotives joined by a common boiler, have a + between the arrangements of each engine. Thus a "double Pacific" type Garratt is a. For Garratt locomotives the + sign is used even when there are no intermediate unpowered wheels, e.g. the LMS Garratt. This is because the two engine units are more than just power bogies. They are complete engines, carrying fuel and water tanks. The + sign represents the bridge (carrying the boiler) that links the two engines.

Simpler articulated types such as Mallets have a jointed frame under a common boiler where there are no unpowered wheels between the sets of powered wheels. Typically, the forward frame is free to swing, whereas the rear frame is rigid with the boiler. Thus a Union Pacific Big Boy is a ; four leading wheels, one group of eight driving wheels, another group of eight driving wheels, and then four trailing wheels.

Duplex locomotives
This numbering system is shared by duplex locomotives, which have powered wheel sets sharing a rigid frame.

Suffixes
A number of standard suffixes can be used to extend the Whyte notation:
 * No suffix means a tender locomotive.
 * T indicates a tank locomotive. This is sometimes extended to indicate the type of tank locomotive:
 * T means a side tank
 * PT a pannier tank
 * ST a saddle tank
 * IST is an inverted saddle tank
 * WT well tank
 * T+T means a tank locomotive that also has a tender
 * VB or VBT mean a vertical boilered locomotive
 * F means a fireless locomotive e.g.
 * R means a rack locomotive, eg
 * G means a geared steam locomotive eg
 * C means a crane tank eg

Internal combustion locomotives
The wheel arrangement of small diesel and petrol locomotives can be classified using the same notation as steam locomotives, e.g. 0-4-0, 0-6-0, 0-8-0. Where the axles are coupled by chains or shafts (rather than side rods) or are individually driven, the terms 4w, 6w or 8w are generally used. Suffixes specific to internal combustion locomotives include:


 * PM Petrol mechanical
 * D Diesel
 * DMDiesel mechanical
 * DE Diesel electric
 * DH Diesel hydraulic
 * BE Battery electric
 * OE Overhead electric lines
 * CA Compressed air

Thus indicates a four-wheel petrol locomotive with mechanical transmission and  indicates a six-wheeled diesel locomotive with side rods and an electric transmission. For large diesel locomotives the UIC classification is used.

Naming
In American (and to a lesser extent British) practice, most wheel arrangements in common use were given names, sometimes from the name of the first such locomotive built. For example, the 2-2-0 type arrangement is named Planet, after the 1830 locomotive on which it was first used. (This naming convention is similar to the naming of warship classes.)

Common wheel arrangements
The most common wheel arrangements are listed below. In the diagrams, the front of the locomotive is to the left.