This emerged in the Railways Act 1921, which was to establish four new large railway companies, the "groups", and most of the railways of Great Britain would be compulsorily restructured into them, in a process known as the "grouping". There would be a new Great Western Railway; the old GWR was naturally the largest constituent of it, but the Barry Railway too was a constituent company, the fifth largest in capitalisation, larger only than the Rhymney Railway. The change was effective from 1 January 1922; £100 of Barry Railway ordinary stock was converted into £220 of Great Western Railway 5% stock.