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.
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. These small railway companies kept their independence.