Work to upgrade rail services in South Wales, including the Valley Lines, has been completed.
The £300 million re-signalling project affected services across the festive period with many trains replaced by buses.
Network Rail and Arriva Trains Wales have thanked passengers for their patience.
As part of the project, Network Rail replaced outdated signalling with new technology, upgraded the track layout at key points on the Cardiff and Valleys network, and delivered new platforms and improved stations.
Platform 8 at Cardiff Central has also opened for passenger use: providing an enhanced new waiting area for passengers and allowing greater flexibility when running trains in and out of the city centre.
Andy Thomas, Wales Route Managing Director, said: “I’m incredibly grateful to passengers for bearing with us while we delivered this vital upgrade.
“This work is a cornerstone of our Railway Upgrade Plan to build a bigger, better railway here in Wales.
“This is the largest and most complex signalling phase delivered to date anywhere in Britain, unlocking the potential to run 16 trains per hour – from 12 – into the heart of our capital city, should fleet be made available in the future. It has also increased operational resilience and our capacity to recover when incidents arise in the meantime.”
Ian Bullock, managing director of Arriva Trains Wales, said: “We would like to thank everyone travelling for their patience over the festive period whilst the improvement works have been taking place.
“It is fantastic to have the brand new Platform 8 operational for the Barry Island, Penarth and Vale of Glamorgan services. The new modern facilities will definitely enhance customer experience and help to ease congestion at the station.”
Christian Irwin, programme manager at Network Rail Wales said: “I’m very thankful for the patience shown by passengers in South Wales over the festive period.
“Although I understand disruption is never ideal, the work we have delivered will benefit rail users for many decades to come and I’m really proud of what we’ve achieved.”