When it was originally published, Hugh Longworth''s definitive record of every steam locomotive operated by British Railways from nationalisation until the end of steam traction on the network in 1968 was very much welcomed as it provided for the first time in one place a detailed and accurate source of reference on this important subject. With the book having been out of print for some time, a new and revised edition was overdue. However, the extent of the revisions, which included the book''s many photographs being given more space on the page and the use of enlarged and enhanced locomotive drawings, greatly increased the size of the work. Rather than attempt to cut or dilute the content, it was decided that instead of publishing one unwieldy and expensive book, a more realistic approach was to present the work in two volumes of roughly similar size. This is the first of these and focuses on the GWR and LNER locomotives which became part of BR stock in 1948. The second book, due towards the end of 2025, will cover Southern and LMS locos and the Standard classes introduced by BR. The entry for each class provides essential information including the size of cylinders and driving wheels, boiler pressure, weight, the type of valve gear used, the year the class was introduced and its tractive effort. For each locomotive in a class, its date and place of construction and withdrawal and scrapping dates are given, as are details of any renumberings that may have occurred during the BR era. Heavily illustrated throughout, this book and its companion volume will be of great value to all interested in the steam era on British Railways. All classes that were on BR''s books in the years post 1948 are covered, and whilst the overwhelming majority of the locos featured are standard gauge, the few narrow gauge locomotives operated by BR are also included.