Whyte notation describes the arrangement of wheels on a steam engine. It uses 3 numbers separated by dashes. In order, it shows the number of uncoupled wheels in front of the driving wheels, the number of coupled driving wheels, and the number of uncoupled wheels behind the driving wheels (ignoring wheels on the tender if the locomotive has one).
A letter based suffix is used to show whether the locomotive is a tender engine or tank engine:
- No suffix indicates a tender engine.
- T indicates a ‘side tank’ engine, with a tank each side of the boiler resting on the locomotive’s frames.
- PT indicates a ‘pannier tank’ engine, having a tank hung on each side of the boiler but clear of the frames. This arrangement was used most commonly, though not exclusively, by the GWRGreat Western Railway.
- ST indicates a ‘Saddle tank’ engine, having a single tank slung over the boiler like a saddle.
- WT indicates a 'Well tank' engine, having the water tank between the frames, under the boiler.
Thus ‘Flying Scotsman’ would be described as a 4-6-2 (a tender engine with 4 leading wheels, 6 coupled driving wheels and 2 trailing wheels), while ‘Thomas the tank engine’ is correctly described as 0-6-0T (6 coupled driving wheels, no leading or trailing wheels, and side tanks).
Certain common configurations acquired names, those most relevant to the SVRSevern Valley Railway being:
- 2-6-0 MogulLocomotive with a 2-6-0 wheel configuration (SVRSevern Valley Railway domiciled examples include 43106 and 7325)
- 2-6-2 PrairieLocomotive with a 2-6-2 wheel configuration (SVRSevern Valley Railway domiciled examples include 4566 and 82045)
- 4-6-2 PacificLocomotive with a 4-6-2 wheel configuration
Thus ‘Flying Scotsman’ would normally be referred to as an example of a GresleySir Nigel Gresley, Chief Mechanical Engineer (CME) of the London & North Eastern Railway 1923-1941 PacificLocomotive with a 4-6-2 wheel configuration, rather than a GresleySir Nigel Gresley, Chief Mechanical Engineer (CME) of the London & North Eastern Railway 1923-1941 4-6-2.
Small diesel and petrol engine locomotives may be described using the same system. This may be followed by D for diesel or P for petrol, and another letter describing the transmission: E for electric, H hydraulic, M mechanical. Thus 0-6-0DM denotes a six-wheel diesel locomotive with mechanical transmission.
For large diesel locomotives the UIC classification is used. UICUnion Internationale des Chemins de fer (International Union of Railways) classification can also be used to describe steam locomotives, although in the UK Whyte notation has prevailed apart from the brief use of a modified version of UICUnion Internationale des Chemins de fer (International Union of Railways) classification by the Southern Railway described here.