Railfans keen on steam will no doubt be aware of the dazzling array of wheel arrangements that adorn a wide range of locomotives. From tank engines and narrow gauge locomotives, to large freight haulers and those pulling high speed express trains, each and every type of steam locomotive operates differently and has a wheel set to match its purpose. In 1900, mechanical engineer Frederick Methvan Whyte devised a relatively simple form of notation to describe wheel arrangements. Below we take a look at how Whyte Notation is used so that railfans can understand what is meant by these numerical designations.
The three types of wheels: The first thing to recognise is that steam locomotives have three basic types of wheels; Leading Wheels, Driving Wheels and Trailing Wheels. All locomotives have Driving Wheels but not all have leading or trailing wheels.
Leading Wheels are not powered and sit in front of the Driving Wheels to help support the front of the locomotive's boiler and / or guide the locomotive round curves and switches / points. The number of Leading Wheels is signified by the first number in Whyte Notation.
Driving Wheels are those that actually move the locomotive through either motors (Diesel / Electric Locomotives) or pistons and rods (Steam Locomotives). With the latter, they more often than not sit centrally underneath the boiler and are larger than other unpowered wheels surrounding them. The number of Driving Wheels is signified by the middle set(s) of numbers in Whyte Notation. Locomotives with multiple sets of driving wheels will have multiples of numbers included in their notation.
Trailing Wheels sit behind the Driving Wheels and often simply support the cab and firebox in large locomotives. Occasionally they are also powered to provide additional traction. The number of Trailing Wheels is denoted by the final number in Whyte Notation.
Whyte Notation is written to represent the number of wheels, with the three types of wheels separated by hyphens "-". As a result, a locomotive with 2 Leading Wheels, 4 Driving Wheels and 2 Trailing Wheels would be described as a 2-4-2. Locomotives without either Leading or Trailing Wheels still have to be accounted for, and so in the example above (top middle) where the locomotive has just 4 Driving Wheels, the designation is 0-4-0.
Important points to note:
The number of wheels on the Tender (with Tender Locomotives) do not count towards the total number of Trailing Wheels. For the purposes of Whyte Notation, Tenders are effectively ignored.
Locomotives that have two or more distinct sets of Driving Wheels have them noted separately in Whyte Notation. For example, the Union Pacific 'Big Boy' has two sets of eight Driving Wheels and is therefore described as a 4-8-8-4 locomotive and not a 4-16-4 locomotive.
Articulated Locomotives, such as Garratts or Fairlies have their two sets of wheels separated by a "+" sign. For example: 0-6-0+0-6-0
Various letters are suffixed to Whyte Notation to denote specific types of Tank Locomotives. For example: 0-6-2T
Despite predating the system, Stephenson's Rocket can be described as an 0-2-2 locomotive, with just 2 Driving Wheels and 2 Trailing Wheels. This illustration is from an Italian book of 1898. Courtesy of the British Library's Flick Collection.This illustration, also from 1898, shows a 4-4-0 locomotive of the Lancashire and Yorkshire Railway. Courtesy of the British Library's Flick Collection.Mike Rusnak sent us this fantastic shot, for a previous We Are Railfans article, of 'Big Boy' 4014 basking in the sun during a visit to Milwaukee in 2019. It clearly shows the wheels in a 4-8-8-4 formation.5900 'Hinderton Hall' is seen here at Didcot Railway Centre, England. This 4-6-0 locomotive saw extensive use in both service and preservation. The photograph is part of an anonymous and previously unpublished collection obtained by We Are Railfans.