The Rhymney Railway (RR) 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.
The first line was dependent on the cooperation of the parallel Taff Vale Railway (TVR) for part of the transit, and this relationship was uneasy; the Rhymney Railway built an independent line to Cardiff in 1871. Better relations were created with the London and North Western Railway (LNWR), and later the Great Western Railway (GWR), and two important joint lines with the GWR were built: the Taff Bargoed line (1876) and the Quakers Yard and Merthyr Joint line (1882).
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.