 From that time a firm alliance was formed, and express passenger trains to London ran over the Dumfries line and via Leeds on the Midland line, reaching London at St Pancras station.The G&SWR expanded south-westward from Glasgow, where more traffic—mineral as much as passenger—was available than in the moorlands of Nithsdale.The G&SWR continued in existence until the "grouping" of the main line railways in Great Britain, under the Railways Act 1921; the G&SWR, together with the Caledonian Railway, became constituents of the London, Midland & Scottish Railway (LMS).