5. c. 55),[1] also known as the Grouping Act, was an act of Parliament enacted by the British government, and was intended to stem the losses being made by many of the country's 120 railway companies, by "grouping" them into four large companies, dubbed the "Big Four".

The form of the act was developed by the Minister of Transport, Eric Geddes, who was a former North Eastern Railway executive.

Geddes favoured using amalgamations to create privately owned regional monopolies, and suggested increased worker participation from pre-war levels.

Geddes viewed the pre-war competition as wasteful, but was opposed to nationalisation, on the grounds that it led to poor management, as well as a mutually corrupting influence between railway and political interests.

The move to greater worker participation was strongly opposed by the RCA, but supported by the Labour Party.

[1] Parts 5 and 6 dealt with light railways and general clauses respectively, with the general clauses of part 6 including the requirement of the railway companies to provide the Minister of Transport with statistic and financial reports.

The House of Lords made various amendments, which were accepted by the Commons on 19 August, and royal assent was given.

The lines which remained independent were principally those under the influence of Colonel Stephens, who had been instrumental in securing the necessary exemption.