Prime Minister David Cameron will unveil the plans today after agreeing a funding package with the Welsh government.

He will also announce that the UK government will cover the full costs of electrifying the Great Western mainline to Swansea and devolve the Wales and Borders rail franchise so that the Welsh government decides the new franchise in 2018.

Cameron said: “After years of neglect, this part of Wales will finally get the infrastructure it needs with faster, more modern, more efficient trains and the impact will be huge.”

Secretary of State for Wales Stephen Crabb said: “I am delighted that we can now seize this opportunity to transform the Valley communities.

“Effective transport links are a vital part of any modern economy and there are few areas in the UK more in need of the improved commuter costs, travel times and more frequent train services that this investment brings.”

Journey times from both Merthyr Tydfil and Treherbert to Cardiff would be reduced to a little over 50 minutes.

The UK government will take over sponsorship and fund delivery of the Cardiff-Bridgend section of the Main Line electrification scheme to Swansea – worth £105m, and contribute £125m towards the costs of the wider Valley Lines electrification scheme.

The Welsh government will take over sponsorship and delivery of the Valley Lines project.