The Merthyr Line is a commuter railway line in South Wales from central Cardiff to Merthyr Tydfil and Aberdare. The line is part of the Cardiff urban rail network, known as the Valley Lines. It is historically the Taff Vale Railway, the first rail development in the Valleys in the 1840s and associated with the notorious Taff Vale Judgment in 1901 when the courts penalised trade unions for losses caused by strikes. The line is currently operated by Arriva Trains Wales (ATW) as part of the Valley Lines network. ATW replaced the previous franchise, Wales & Borders Trains in December 2003.