Whyte notation
The Whyte notation is a classification method for steam locomotives, and some [|internal combustion locomotives] and electric locomotives, by wheel arrangement. It 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.
Method
Basic form
The notation in its basic form counts the number of leading wheels, then the number of driving wheels, and finally the number of trailing wheels, numbers being separated by dashes. For example, a locomotive with two leading axles in front, then three driving axles and then one trailing axle is classified as a locomotive, and is commonly known as a Pacific.Denotion of other locomotives
Articulated locomotives
For articulated locomotives that have two wheelsets, such as Garratts, which are effectively two locomotives joined by a common boiler, each wheelset is denoted separately, with a plus sign between them. Thus a "double Pacific" type Garratt is a. For Garratt locomotives, the plus 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 plus sign represents the bridge 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
For duplex locomotives, which have two sets of coupled driving wheels mounted rigidly on the same frame, the same method is used as for Mallet articulated locomotives – the amount of leading wheels are placed first, followed by the leading set of driving wheels, followed by the trailing set of driving wheels, followed by the trailing wheels, each number being separated by a hyphen.Tank locomotives
A number of standard suffixes can be used to extend the Whyte notation for tank locomotives:| Suffix | Meaning | Example |
| Tender locomotive | ||
| T | Side tank locomotive | |
| ST | Saddle tank locomotive | |
| WT | Well tank locomotive | |
| PT | Pannier tank locomotive | |
| C or CT | Crane tank locomotive | |
| IST | Inverted saddle tank locomotive | |
| T+T | Tank locomotive which also has a tender |
Other steam locomotives
Various other types of steam locomotive can be also denoted through suffixes: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 or are individually driven, the terms 4w, 6w or 8w are generally used. For larger locomotives, the UIC classification is more commonly used.Various suffixes are also used to denote the different types of internal combustion locomotives:
| Suffix | Meaning | Example |
| PM | Petrol-mechanical locomotive | |
| PE | Petrol-electric locomotive | |
| D | Diesel locomotive | |
| DM | Diesel–mechanical locomotive | |
| DE | Diesel–electric locomotive | |
| DH | Diesel–hydraulic locomotive |
Electric locomotives
The wheel arrangement of small electric locomotives can be denoted using this notation, like with internal combustion locomotives.Suffixes used for Electric locomotives include:
| Suffix | Meaning | Example |
| BE | Battery-electric locomotive | |
| OE | Overhead-lines electric locomotive |