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 GWR.
- ST indicates a ‘Saddle tank’ engine, having a single tank slung over the boiler like a saddle.
Thus ‘Flying Scotsman’ would be described as a 4-6-2 (a tender engine with 2 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 SVR being:
- 2-6-0 Mogul
- 2-6-2 Prairie
- 4-6-2 Pacific
Thus ‘Flying Scotsman’ would normally be referred to as an example of a Gresley Pacific, rather than a Gresley 4-6-2.