Wheel and Axle Configuration

Wheel and Axle Configurations

By Roger Heid


Imagine you are standing next to a steam locomotive, looking at it from the side. Depending on its type, you will see a bunch of bigger and smaller wheels.


                                                                                                                                                                                          Front                        Center                          Rear

The smaller wheels on the front are called Pilots or Leaders. The big wheels in the center are called the drive wheels, sometimes coupled wheels. The smaller wheels in the rear are the trailers. The leaders and trailers serve to guide and support the locomotive.

There are two primary notations to describe a given wheel or axle configuration. In the US, there is the ‘Whyte Notation’. In this system, the total of all wheels is counted. The sketch shown above is therefore a 4-6-2.

Germany uses the ‘UIC Notation’. The total of all axles is counted, whereby the drive axles are reflected by using alphabet letters. The 3rd letter in the alphabet is a C. Ergo the sketch above shows a 2C1. Some time ago, a refinement was added to this. If a given wheel assembly is hinged or articulated, it receives an apostrophe behind the number or letter. That makes this one a 2’C1’.

All steam locomotives featuring this configuration are generally referred to as a ‘Pacific’, named after a certain loco introduced in the US in 1901. It was considered a break-through in steam locomotive development, and it was quickly adopted in several other countries.

Now, let us take a look at a BR 01, the most famous of all German Pacifics.


You can clearly see what I am talking about. It is a 4-6-2 or a 2’C1’, depending which notation system you prefer to use.

Obviously, not all steam locomotives feature the same configuration. There are many variants. Below are sample pictures of different configurations.


BR 89  

Whyte:  0-6-0

UIC:  0C0


BR 64

Whyte: 2-6-2

UIC:  1′C1′


BR  78

Whyte: 4-6-4

UIC:  2′C 2′


BR 96 (Mallet)

Whyte:  0-8-8-0

UIC:  0D’D’0

By now you should be able to name the configuration, in either notation, of any steam locomotive you may encounter. The Whyte and UIC notation systems are the most commonly used. Some other countries use slightly different systems, but they are all easily recognized.

If you have any questions, please post them in the Forum. The Blog System is not designed to answer questions.



Union Pacific Big Boy

Whyte:  4-8-8-4

UIC:  2′D’D’2′


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