List of railway companies involved in the 1923 grouping

Under the Railways Act 1921 the majority of the railway companies in Great Britain (along with a few in Northern Ireland) were grouped into four main companies, often termed the Big Four. The grouping took effect from 1 January 1923.

The Big Four and their constituent companies, showing route mileage, were:

• Great Western Railway (GWR)
  • Great Western Railway 3,005 miles (4,836 km)
  • Alexandra (Newport and South Wales) Docks and Railway (ADR) 10.5 miles (16.9 km)
  • Barry Railway (Barry) 68 miles (109 km)
  • Cambrian Railways (Cambrian) 295.25 miles (475.16 km)
  • Cardiff Railway (Cardiff) 11.75 miles (18.91 km)
  • Rhymney Railway (RhyR) 51 miles (82 km)
  • Taff Vale Railway (TVR) 124.5 miles (200.4 km)
    • for the list of subsidiary companies and joint railways see List of constituents of the Great Western Railway
• London and North Eastern Railway (LNER)
  • Great Central Railway (GCR) 852.5 miles (1,372.0 km)
  • Great Eastern Railway (GER) 1,191.25 miles (1,917.13 km)
  • Great Northern Railway (GNR) 1,051.25 miles (1,691.82 km)
  • Hull and Barnsley Railway (H&BR) 106.5 miles (171.4 km)
  • North Eastern Railway (NER) 1,757.75 miles (2,828.82 km)
  • Great North of Scotland Railway (GNSR) 334.5 miles (538.3 km)
  • North British Railway (NBR) 1,378 miles (2,218 km)
    • for the list of subsidiary companies and joint railways see List of constituents of the London and North Eastern Railway
• London, Midland and Scottish Railway (LMS)
  • London and North Western Railway (LNWR); 2,667.5 miles (4,292.9 km)
    • including Lancashire and Yorkshire Railway (L&YR) amalgamated from 1 January 1922
  • Furness Railway (Furness); 158 miles (254 km)
  • Midland Railway (MR) 2,170.75 miles (3,493.48 km)
  • North Staffordshire Railway (NSR) 220.75 miles (355.26 km)
  • Caledonian Railway (CalR) 1,114.5 miles (1,793.6 km)
  • Glasgow and South Western Railway (GSWR) 493.5 miles (794.2 km)
  • Highland Railway (HR) 506 miles (814 km)
    • for the list of subsidiary companies and joint railways see List of constituents of the London, Midland and Scottish Railway
  • The Railways Act 1921 did not extend to Ireland, but Irish lines owned by constituent companies became part of the LMS:
    • Dundalk, Newry and Greenore Railway (DNGR) 26.5 miles (42.6 km) (owned by the LNWR)
    • Northern Counties Committee lines (NCC) 265.25 miles (426.88 km) (owned by the Midland Railway)
      • The NCC and Great Northern Railway (Ireland) (GNR(I)) operated the County Donegal Railways Joint Committee lines jointly, and these became joint lines of the LMS and GNR(I) after grouping.
• Southern Railway (SR)
  • London, Brighton and South Coast Railway (LBSCR) 457.25 miles (735.87 km)
  • London and South Western Railway (LSWR) 1,020.5 miles (1,642.3 km)
  • South Eastern and Chatham Railways' Managing Committee: a working union of the South Eastern Railway and the London, Chatham and Dover Railway 637.75 miles (1,026.36 km)
    • for the list of subsidiary companies and joint railways see List of constituent companies of the Southern Railway

Over fifty railway companies were not covered by the grouping. These small railway companies kept their independence.