BR standard classes

The steam locomotives of British Railways were used by British Railways over the period 1948–1968. The vast majority of these were inherited from its four constituent companies, the "Big Four".

In addition, BR built 2537 steam locomotives in the period 1948–1960, 1538 to pre-nationalisation designs and 999 to its own standard designs. These locomotives had short working lives, some as little as five years, because of the decision to end the use of steam traction by 1968, against a design life of over 30 years and a theoretical final withdrawal date of between 1990 and 2000.

British Railways was created on 1 January 1948 principally by the merger of the "Big Four" grouped railway companies: the Great Western Railway (GWR), the London, Midland and Scottish Railway (LMS), the London and North Eastern Railway (LNER) and the Southern Railway (SR). It inherited a wide legacy of locomotives and rolling stock, much of which needed replacing due to the ravages of World War II.

A wide variety of locomotives was acquired from the four major constituent companies. These had generally standardised their own designs. See:

In addition, a handful of locomotives were inherited from minor constituents.

The 1948 Locomotive Exchange Trials compared locomotives from each company against each other.

After initially using letter prefixes (E for ex-LNER, M for ex-LMS, S for ex-SR, and W for ex-GWR locomotives, as used for other inherited rolling stock), a numbering scheme was decided on in March 1948. Generally ex-GWR locomotives retained their numbers (and hence were able to retain their cast brass number plates) and it was decided to add 30000 to the Southern numbers, 40000 to the LMS numbers and 60000 to the LNER numbers. There were some exceptions though.

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