The Rhymney Railway was a railway company in South Wales, founded to transport minerals and materials to and from collieries and ironworks in the Rhymney Valley of South Wales, and to docks in Cardiff. It opened a main line in 1858, and a limited passenger service was operated in addition. Although the Rhymney Railway network was never large, it was remarkably profitable, and paid excellent dividends for most of its life. Dependent on mineral traffic for its own success, it declined in the 1970s, but the main line from Rhymney to Cardiff remains in heavy use as a local passenger line.