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Thus, a locomotive with two leading axles (and thus four wheels) in front, then three driving axles (six wheels) and followed by one trailing axle (two wheels) is classified as a 4-6-2.

It's important to stress that wheels, not axles, are what is counted in this system. Other classification schemes in use elsewhere (such as in France) count axles.

The system had to be extended with the advent of articulated locomotives. The scheme generally adopted is that locomotives such as Garratts, where there are, in effect, two separate locomotives joined by a common boiler, are classified by using a plus sign in between the arrangements of each engine. Thus, a 'double Pacific' type Garratt is a 4-6-2+2-6-4.

Simpler articulated types such as Mallets, where there are no unpowered axles in between powered axles, are just written by adding extra numbers in the middle; each number represents a grouping of wheels. Thus a Big Boy is written under this modified Whyte notation as a 4-8-8-4; there are two leading axles, one group of four driving axles, another group of four driving axles, and then two trailing axles.

In addition the suffix 'T' is often used to indicate a tank locomotive (otherwise, a tender locomotive is assumed). In British practice, this is sometimes extended to indicate what type of tank locomotive. When this is done, a plain 'T' means side tank, 'ST' means saddle tank, PT means pannier tank and WT stands for well tank. Where a 'T' suffix is followed by '+T', this indicates a tank locomotive that has a tender for additional coal or water capacity.

The limitations of the Whyte system for classifying locomotives that did not fit the standard steam locomotive pattern led to the design of other forms of classification. Most commonly used in Europe is the UIC classificationThe UIC classification is a comprehensive system for describing the wheel arrangement of a locomotive. It is the system generally used in much of the world, but it was not historically used in the United Kingdom or United States; a simplified form is used scheme, based on GermanThe Federal Republic of Germany ( German: Bundesrepublik Deutschland is one of the world's leading industrialized countries, located in the middle of the European Union. It is bordered to the north by the North Sea, Denmark and the Baltic Sea, to the east practice, which can more completely define the exact layout of a locomotive.

In AmericanThe United States of America also referred to as the United States U. America ¹ or the States is a federal republic in central North America, stretching from the Atlantic in the east to the Pacific Ocean in the west. It shares land borders with Canada in (and to a lesser extent BritishThe United Kingdom of Great Britain and Northern Ireland is a state in Western Europe, usually known simply as the United Kingdom the UK Britain or less accurately as Great Britain . The UK was formed by a series of Acts of Union which united the formerly) practice, most wheel arrangements in common use were given names.

Here is a list of the most common wheel arrangements: in the illustration, which should be read left to right, with the front of the locomotive to the left, small o is a carrying axle, and a big O is a driving axle.