Companies were defined as constituent companies if they were major elements in the grouping, being merged or amalgamated with one another, or subsidiary companies if they were minor companies that could be acquired rather than given the status of being ‘merged’. In anticipation of the grouping, there were a number of acquisitions of smaller railways in the year or so before the grouping took effect on 1 January 1923.
The Railways Act 1921 defined the new shape of the railways as consisting of a ‘Southern Group’, a ‘Western Group’, a ‘North Western, Midland, and West Scottish Group’, and a ‘North Eastern, Eastern, and East Scottish Group’, which, of course, became the Southern, Great Western, London Midland Scottish and London & North Eastern respectively.
This meant that the Great Western Railway had as its constituent companies the original GWR itself, plus the Barry Railway; Cambrian Railway; the Rhymney Railway; the Taff Vale Railway and the Alexandra (Newport and South Wales) Docks & Railway; with, as subsidiary companies, the Brecon & Merthyr Tydfil Junction Railway; Burry Port & Gwendraeth Valley Railway; Cleobury Mortimer & Ditton Priors Light Railway; Didcot Newbury & Southampton Railway; Exeter Railway; Forest of Dean Central Railway; Gwendraeth Valleys Railway; Lampeter Aberayron & New Quay Light Railway; Liskeard & Looe Railway; Llanelly & Mynydd Mawr Railway; Mawddwy Railway; Midland & South Western Junction Railway; Neath & Brecon Railway; Penarth Extension Railway; Penarth Harbour Dock & Railway; Port Talbot Railway & Docks; Princetown Railway; Rhondda & Swansea Bay Railway; Ross & Monmouth Railway; South Wales Mineral Railway; Teign Valley; Van Railway; Welshpool & Llanfair Light Railway; West Somerset Railway, and the Wrexham & Ellesmere Railway.
The London & North Eastern Railway’s constituent companies were the Great Central Railway; Great Eastern Railway; Great North of Scotland Railway; Great Northern Railway; Hull & Barnsley Railway; North Eastern Railway and the North British Railway; while as subsidiaries there were the Brackenhill Light Railway; Colne Valley & Halstead Railway; East & West Yorkshire Union Railway; East Lincolnshire Railway; Edinburgh & Bathgate Railway; Forcett Railway; Forth & Clyde Junction Railway; Gifford & Garvald Railway; Great North of England Railway; Clarence & Hartlepool Junction Railway; Horncastle Railway; Humber Commercial Railway & Dock; Kilsyth & Bonnybridge Railway; Lauder Light Railway; London & Blackwall Railway; Mansfield Railway; Mid-Suffolk Light Railway; Newburgh & North Fife Railway; North Lindsey Light Railway; Nottingham & Grantham Railway; Nottingham Joint Station Committee; Nottingham Suburban Railway; Seaforth & Sefton Junction Railway; Stamford & Essendine Railway and the West Riding Railway Committee.
The London Midland Scottish had as its constituent companies the Caledonian Railway; Lancashire & Yorkshire Railway, which had already reached an agreement to be purchased by the London & North Western Railway; Glasgow & South Western Railway; Highland Railway; the Midland Railway; North Staffordshire Railway and the Furness Railway; with subsidiaries including the Arbroath & Forfar Railway; Brechin & Edzell District Railway; Callander & Oban Railway; Cathcart District Railway; Charnwood Forest Railway; Cleator & Workington Junction Railway; Cockermouth Keswick & Penrith Railway; Dearne Valley Railway; Dornoch Light Railway; Dundee & Newtyle Railway; Harborne Railway; Leek & Manifold Valley Light Railway; Maryport & Carlisle Railway; Mold & Denbigh Junction Railway; North & South Western Junction Railway; North London Railway; Portpatrick & Wigtownshore Joint Committee; Shropshire Union Railways & Canal; Solway Junction Railway; Stratford-upon-Avon & Midland Junction Railway; Tottenham & Forest Gate Railway; Wick & Lynster Light Railway; Wirral Railway and the Yorkshire Dales Railway.
As the smallest of the ‘Big Four’ grouped companies, the Southern Railway consisted of three constituent companies, the London Brighton & South Coast Railway; the London & South Western Railway and the South Eastern & Chatham Railway Companies Managing Committee, itself representing two companies, the South Eastern Railway and the London, Chatham & Dover Railway, that still retained their own assets and shareholders. The subsidiaries were the Bridgwater Railway; Brighton & Dyke Railway; Freshwater, Yarmouth & Newport (Isle of Wight) Railway; Hayling Railway; Isle of Wight Central Railway; Isle of Wight Railway; Lee-on-Solent Railway; London & Greenwich Railway; Mid Kent Railway; North Cornwall Railway; Plymouth & Dartmoor Railway; Plymouth, Devonport & South Western Junction Railway; Sidmouth Railway and the Victoria Station & Pimlico Railway.
For each of the ‘Big Four’, many of the minor companies were already operated by or managed by the constituent companies.