Arriva Trains Wales will run the service every half hour
A new half-hourly train service between Merthyr Tydfil and Cardiff which aims to get commuters out of their cars is to start.
Trains previously travelled to and from the Welsh capital to the valleys town once every hour.
The new service will run at 26 and 56 minutes past the hour from Cardiff and eight and 38 minutes past from Merthyr.
It comes after the assembly government invested £19m in new track and signalling along the busy route.
Work included improving stations at Abercynon in the Cynon Valley and at Merthyr Vale, while upgrading tracks and signalling.
A new park and ride facility at Abercynon is to follow the rail improvements.
Minister for Economy and Transport, Ieuan Wyn Jones will board the 0656 BST train from Cardiff Central to launch the new service, arriving at Merthyr shortly after 0800 BST.
He said the launch was "another significant step" towards delivering a "system of sustainable transport fit for a 21st century Wales".
"Improving the quality and regularity of this important service will encourage many more commuters to use public transport," he said.
"Not only will this help reduce traffic on the roads, but also improve access to jobs, education and training opportunities."
Mike Bagshaw, commercial director Arriva Trains Wales, which will run the service, said: "From today we are able to offer our passengers a more flexible convenient service which will boost the local economy and future regeneration of Merthyr."
Mike Gallop, route enhancement manager for Network Rail said its engineers had invested more than 70,000 hours to design and build the infrastructure over the last two years.
He said more improvements were scheduled for other towns in the south Wales valleys.
"It marks the first of a series of improvements that is set to be delivered to all key valley towns over the next five years to bring more trains to meet the burgeoning passenger growth in south Wales," he added.