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.

Urban and suburban rail plays a key role in public transport in many of the major cities of the United Kingdom. Urban rail refers to the train service between city centres and suburbs or nearby towns that acts as a main mode of transport for travellers on a daily basis. They consist of several railway lines connecting city centre stations of major cities to suburbs and surrounding towns.