Class (locomotive) refers to a group of locomotives built to a common design for a single railroad. Often members of a particular class had detail variations between individual examples, and these could lead to subclasses. Sometimes technical alterations (especially rebuilding, superheating, re-engining, etc.) move a locomotive from one class to another. Different railways had different systems, and sometimes one railway (or its successors) used different systems at different times and for different purposes, or applied those classifications inconsistently. Sometimes therefore it is not clear where one class begins and another ends. The result is a classic example of the Lumper splitter problem.
1 Development 2 Class system organization
2.1 First level: Wheel arrangement 2.2 Second level: Sequential model 2.3 Third level: Variants
3 Class names 4 British practice
4.1 Great Western Railway 4.2 Southern Railway 4.3 London, Midland and Scottish Railway 4.4 London and North Eastern Railway
5 German practice 6 Japan 7 See also
As locomotives became more numerous the need arose to deal with them
in groups of similar engines rather than as named or numbered
individuals. These groups were named "classes" and at first tended to
reflect capability rather than design. For example, the Baltimore and
Ohio Railroad grouped its roster into four classes before the Civil
War, though they had by that point dozens of different designs.
Later classes were based on design. A group of locomotives built off
the same blueprints constituted a class, and if some of the
locomotives in the class were sufficiently modified, a new class might
be established for the modified examples. When electric locomotives
were introduced, the same scheme was applied to them.
Since steam and early electric locomotives were usually custom built,
classes were assigned by the railroad, and each railroad had its own
system. Mergers of lines and sales of locomotives brought about
changes of class. Early diesels were often fitted into the locomotive
class system, but since they were generally not custom built the use
of manufacturer model designations overtook the class system and made
it irrelevant, except for historical discussion.
Class system organization
Usually the class system for a railroad was built on a simply
hierarchy which assigned each class a code.
First level: Wheel arrangement
The first level was usually for the wheel arrangement and was usually
coded by a letter of the alphabet. Different railroads used different
codes, so that "J" on the
New York Central Railroad
Union Pacific Big Boy
For recent practice see: British Rail locomotive and multiple unit numbering and classification
The old railway companies had various systems of classification. Taking the "Big Four" companies which operated from 1923 to 1947: Great Western Railway The class number was usually taken from the first member of each class, e.g. "5700 Class" or "57XX Class" for locomotives in the number series beginning 5700.
See: GWR locomotive numbering and classification
Southern Railway Each class was given a letter or number but these were not very meaningful. For example, "700" Class locomotives were 0-6-0s, but so were "Q" Class engines. See:
SR locomotive numbering and classification SR multiple unit numbering and classification
London, Midland and Scottish Railway Each locomotive was given a power classification, e.g. "3F". However, many different classes would have the same power classification so this was not helpful for identifying classes.
See: LMS locomotive numbering and classification
London and North Eastern Railway The LNER's classification system was the most helpful to railway enthusiasts. Each wheel arrangement was given a letter (e.g. A for 4-6-2, B for 4-6-0, etc.) and this was followed by a number denoting the class (e.g. A1, B1, etc.).
See: LNER locomotive numbering and classification
German practice Main article: German steam locomotive classification Japan Main article: Japan Railways locomotive numbering and classification See also
Baltimore and Ohio Railroad