The below text is from Wikipedia’s entry for the Steam Locomotives of British Railways

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 2,537 steam locomotives in the period 1948–1960, 1,538 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

Background

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.

Locomotives acquired from the War Department

In addition to the inherited and new-build locomotives, B.R. also purchased 620 locomotives of three types from the War Department. These had been in use on railways in Great Britain and elsewhere in Europe during the Second World War. For two of these types, B.R. was adding to two classes it already had. B.R. had inherited 556 ex-LMS Stanier Class 8F 2-8-0s, and added 39 in 1949 and an additional three in 1957, bringing the class total to 666. Additionally, it had acquired 200 ex-LNER Class O7 2-8-0s of the WD Austerity 2-8-0 type, to which it added another 553 examples. The ex-LNER locomotives were later renumbered from the ex-LNER 6xxxx series into the BR series as 90533–732. The third type, of which it had no other examples, were the 25 of the WD Austerity 2-10-0s. Of the eight WD ex-LMS Fowler Class 3F 0-6-0Ts exported to France, the five survivors were repatriated in 1948, and resumed their original numbers in the sequence of LMS Fowler Class 3F locomotives (albeit with the additional 40000 that identified ex-LMS locomotives under BR ownership). The ex-WD Hunslet Austerity 0-6-0STs were ex-LNER Class J94 locomotives and are included in the total of LNER locomotives inherited