British Rail coach designations

The London, Midland and Scottish Railway and the London and North Eastern Railway both developed a system of identifying carriages by means of alphabetic codes. When British Railways was formed in 1948, it decided to adopt the former LNER method of carriage classification.
Basic principles.
The codes are made up from a combination of letters, some of which can indicate more than one word; their meaning can only be determined according to their position, or the presence of other letters, in the code. The letters used are:
These letters (except for "Y" and "Z") did not usually apply to the wide variety of passenger-rated but goods carrying vans (e.g. parcels vans, horse boxes, milk and fish vans). Their codes were an acronym of their traditional railway description, e.g. GUV for General Utility Vans.
List of codes used.
The following list lists those codes that were actually used on British Railways, cross-referred to the comparable code used by the LMS, with the exception that the letter "S" (""Second", later "Standard") is used where until 1956 the letter "T" ("Third") is used. Suffix codes "Y" or "Z" are not shown, as these could apply to variants of any or all vehicle types.
In the original LNER coding system, "S" stood for "Second", an intermediate class between "First" and "Third" (which later became "Second"). The original "Second" was more or less abolished in the 1870s (as a result of the Railway Regulation Act 1844), remaining only in limited use for special services, such as those meeting ships (which retained the three-class system from which railway classifications had originated). In the 1980s, BR renamed "Second" to "Standard". Many of the classifications listed below are no longer used, and some did not survive until the designation "Standard"".
Note that in modern usage, composite semi-open vehicles are classified "Cso" – the "s" signifying that the Standard portion is open (implying that the First portion is compartments with corridor).