This page describes the various systems used from time to time to describe the way in which the wheels are distributed under a locomotive.
Modern Diesel and Electric Locomotives
Starting with modern equipment and the usual method of describing how the driving and non-driving (carrying or trailing) wheels are distributed under a locomotive, there are two simple basic rules. First, the wheels are not individually identified, only the axles and, second, trailing wheels are allocated numbers and driving wheels are allocated letters. The letter or number refers to the number of axles in a single frame, for example:
A locomotive with
two bogies, each bogie having two axles is a Bo-Bo or a B-B. Both are shown in this
diagram. The difference between a Bo-Bo and a B-B is that the two axles in a B bogie
are coupled together, either by a coupling rod (once common but now obsolete) or because
they are both driven by the same motor (example).
Unfortunately, over the years, some confusion has arisen because there have been a number
of variations in the way these notations have been used.
On continental Europe, it is common to see a Bo-Bo described as a Bo' Bo'. The - (hyphen) which used to denote the separation between one bogie and the next under a locomotive has been replaced by a ' (apostrophe). The ' is used to denote a swiveling bogie, independent of the frame of the locomotive. This system is even used to describe the axles under a multiple unit train, for example:
2' 2' Bo' Bo' 2' 2'
which is a 3-car set with a leading trailer car, a centre motor car with all axles driven and a rear trailing car.
Further confusion has arisen because of the French habit of describing their locomotive classes with, for example, BB as a prefix to the locomotive number, regardless of whether the locomotive was a B' B' or a Bo' Bo'. They did the same with the CC notation. As the B' B' was very common in France it caused a lot of confusion unless you were an expert on their traction drive systems. The same problem has appeared in the US where the Bo-Bo or Co-Co arrangement is common but they are widely referred to as B-Bs or C-Cs.
One other feature is the use of a + instead of the - between the bogie number or letter. This would indicate the bogies are coupled or articulated in some way.
The 2-Bo Bo-2 is one which offers a strange arrangement. This is from an Italian design of 1935 where the 2-Bo was mounted under a "demi frame" attached to the locomotive at its rear end. Its Italian classification was written as (2' Bo) (Bo 2'). For those interested it was a class E 428 3,000 V DC electric locomotive.
The next example is a 1-D-1, the D referring to the four coupled driving axles in the centre of the locomotive. This was a 3-phase locomotive from 1922. I have not found a design of locomotive with greater than E (five) coupled axles under one frame.
The next two are more recent UK diesel locomotive designs, the 1-Co-Co-1 first seen in 1948 and later on the Class 40s and 45s circa 1960 and the A1A-A1A (Class 31) introduced in 1957. The latter design was adapted from a once popular design of the US.
One rare design seen in the UK was the Co-Bo, designed by Metropolitan Vickers. It was more an exercise in weight distribution than anything else. The non-standard design made it less attractive as a cost effective solution.
Different systems for denoting steam loco wheel arrangements were developed in different countries. In the US and UK it was usual to refer to a steam locomotive type by its wheels rather than its axles. The wheel layout was described totally numerically by first the leading carrying wheels, then the coupled wheels (including the driving wheels) and finally the trailing carrying wheels, in that order, in a system invented by Frederic M Whyte in the US in 1900 e.g. 4-4-0 = ooOO, 4-6-2 = ooOOOo, 0-4-2 = OOo, 0-6-0 = OOO, 2-10-2 = oOOOOOo. A "T" at the end of a description e.g. 0-6-0T, indicated a tank engine, i.e. one not requiring a tender. A good list of steam locomotive wheel arrangements is available at the Webville and Hypertext R.R. site.
Some European railways used a modified form of the Whyte system where the number of axles was used instead of the number of wheels, 4-6-2 becoming 231. This was further developed by the French who used numbers for non-driven axles and letters for driven axles thus; 2C1. This was rearranged by British locomotive designer Bullied who who placed the non-driven axles first in the order, then the driven axles, thus 21C. From the French system it can be seen how the non-steam locomotive axles notation system was developed. For a full list of the French and German systems of wheel notation compared with the US/UK version, see our Steam Locomotive Wheel Arrangements page.