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Locomotive wheel arrangement notation

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A Locomotive wheel arrangement notation is a convention for describing the wheel arrangements of railway locomotives. There are several different notations in use, with the most popular in English-speaking countries being the Whyte system, used mainly for steam locomotives, and for other locomotives, the UIC system used in Europe and the AAR system in the United States.

Both systems indicate the sequence of powered and unpowered wheels (Whyte) or axles (UIC and AAR).

Whyte system

The Whyte system uses a series of numbers separated, in most cases, by hyphens. The first and last numbers are the number of unpowered wheels at the front and rear of the locomotive respectively. Other numbers between the first and last indicate the number(s) of powered wheels coupled together.

A common arrangement is a 4-6-2, which has four unpowered wheels at the front, six driving wheels, and two unpowered wheels at the rear.

Garratt and Meyer locomotives, which have two independent drive units on each locomotive, have two lots of three numbers. The first Garratt ever built is classified as 0-4-0+0-4-0. That is, each unit has four driving wheels and no leading or trailing unpowered wheels.

Mallet and Double Fairlie engines also have two independent drive units on each locomotive, but never had unpowered wheels between the driving wheels. The first Fairlie had an 0-4-4-0 wheel arrangement—two sets of four driving wheels, with no unpowered wheels at either end.

Tank locomotives—those that had the water tank on the locomotive rather than a separate tender towed behind the engine—have the addition of a letter T, or, the following letters to indicate different types of tanks:

  • ST: Saddle Tank
  • PT: Pannier Tank
  • WT: Well Tank

The Whyte system also tends to be used for diesel locomotives that have the driving wheels mechanically coupled together like a steam engine. In these cases, the following letters are sometimes found after the numbers:

  • DH: Diesel Hydraulic
  • DM: Diesel Mechanical

AAR and UIC systems

In the AAR and UIC systems, numbers are used for unpowered axles, and capital letters for powered axles, where A = 1, B = 2, etc. The addition of a small letter "o" indicates that the powered axles each have their own motors.

The UIC system uses an apostrophe at the end of each swivelling bogie, whilst the AAR system uses a hyphen to separate groups of driving wheels on separate bogies. Sometimes the two systems are mixed.

So a Bo'Bo' has two bogies, each with two axles, with each axle independently powered. In the AAR system, this would be described as a Bo-Bo.

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