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:

Companies not covered by the grouping

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:

Railways not included in group companies

Electric or electric and steam lines

Light and similar railways (standard gauge)

Light and similar railways (narrow gauge)

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:

Miscellaneous railways

The railways included in this section were standard gauge, unless otherwise noted: