South Wales Railway

The railways of Great Britain were "grouped" under the terms of the Railways Act 1921, effective from 1 January 1923 (although some effective dates were shortly before or after that date). The pre-1923 Great Western Railway, the Taff Vale Railway, the Rhymney Railway, the Alexandra (Newport & South Wales) Docks & Railway, the Barry Railway, the Cambrian Railways and the Cardiff Railway were constituents of the post-1923 Great Western Railway. Most other lines in South Wales and Gloucestershire west of the River Severn were "subsidiaries" of the Great Western Railway, although some lines that had become owned by the Midland Railway and the London and North Western Railway became part of the new London, Midland and Scottish Railway.

The combined Great Western Railway brought most of the branch lines under GWR control, and enabled efficiencies of scale to be introduced in time. The GWR had long been accused of exploiting its near-monopoly of long-distance rail connection, and the grouping enhanced that hostility:

In South Wales some of the troubles that were to beset the railways were attributed to the Great Western "takeover". It was necessary more than once, to remind business interests, and other people in South Wales, that the amalgamations of 1922 were no result of Great Western aggrandisement. The grouping was imposed by law...

The size of the enlarged company may be judged from the fact that the GWR was responsible for 11% of the local rates of Cardiff in 1924.

The South Wales Railway route now assumed its destiny as the spine of the GWR system, with the former independent railways forming branches off it.

The Development (Loan Guarantees and Grants) Act 1929 (20 & 21 Geo. 5. c. 7) was passed to encourage capital works by the railways and other industries, with the aim of reducing unemployment. The Great Western Railway took advantage of the government assistance, in improving and extending the goods marshalling facilities at Severn Tunnel Junction, and in modernisation and improvements at Cardiff General and Swansea High Street stations. A major power signalling system was installed at Cardiff, and some rationalisation of the duplicate routes at Briton Ferry and Court Sart, a legacy of the Rhondda and Swansea Bay Railway, were undertaken. All this work was done in the period 1930 to 1934.