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

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)
    • 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)

Companies not covered by the grouping

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


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