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:
Over fifty railway companies were not covered by the grouping. Those in the following list were those organised on an independent basis, usually providing locomotives and rolling stock also. They are included under classification headings.
Joint railways
Joint lines in this respect were wholly owned by two or more other companies. If not all of the owning companies went into the same group then the joint company could not be grouped. Joint lines did not always operate any services: they owned the track, stations etc. and the services were operated by one or more of the parent companies:
These are those in which the group companies only are concerned:
Cheshire Lines Committee: (GNR, Midland and GCR); now operated under direction of a board of directors appointed by LNER (two-thirds) and LMS (one-third). Length 142 miles (229 km). Rolling stock owned by CLC; locomotive power by LNER.
Midland and Great Northern Joint Railway: (Midland and GNR; subsequent to the grouping the parent companies were the LMS and the LNER). Length 183 miles (295 km). This was the largest joint system in the UK, and many of the services were operated by the joint company itself. In addition, it received substantial traffic from the GNR and MR/LMS, but rather less post-grouping from the LNER (which also owned the competing ex-GER system in East Anglia).
Railways outside the jurisdiction of the Railways Act 1921
The Railways Act 1921 only extended to Great Britain. Railways in Ireland and the Crown Dependencies were not affected unless owned by a railway company in Great Britain. The railways included in this section were standard gauge, unless otherwise noted: