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:

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 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:

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

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