Whyte notation

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 4-6-2-type Garratt is a 4-6-2+2-6-4. For Garratt locomotives, the plus sign is used even when there are no intermediate unpowered wheels, e.g. the LMS Garratt 2-6-0+0-6-2. 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 (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 4-8-8-4: four leading wheels, one group of eight driving wheels, another group of eight driving wheels, and then four trailing wheels. Sometimes articulated locomotives of this type are denoted with a “+” between each driving wheels set (so in the previous case, the Big Boy would be a 4-8+8-4). This may have been developed to distinguish articulated and duplex arrangements; duplex arrangements would get a “-“ being rigid and articulated locomotives would get a “+” being flexible. However, given all the wheel arrangements for duplex locomotives have been mutually exclusive to them, it is usually considered unnecessary and thus another “-“ is usually used.

Triplex locomotives, and any theoretical larger ones, simply expand on basic articulated locomotives, for example, 2-8-8-8-2. In the case of the Belgian quadruplex locomotive, the arrangement is listed as 0-6-2+2-4-2-4-2+2-6-0.^[4]

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 number of leading wheels is 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:^[5]

Other steam locomotives

Various other types of steam locomotive can be also denoted through suffixes:^[5]

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 (4-wheeled), 6w (6-wheeled) or 8w (8-wheeled) 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:^[5]

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: