The Rhymney Railway was virtually a single stretch of main line, some fifty miles in length, by which the Rhymney Valley was connected to the docks at Cardiff in the county of Glamorgan. The company was incorporated on the 24th July 1854, and the first stretch of line - from Rhymney to Hengoed - was opened in January 1858. The company continued to expand until it was constituted into the Great Western Railway in 1922.