New electric trains have been introduced on the Penarth line.
Transport for Wales (TFW) says five 'tri-mode' trains are now being run on services linking the resort with Coryton and Caerphilly.
The Class 756 trains - which can switch to diesel or battery power on non-electricfied routes - have already been used on the Merthyr, Aberdare and Trehebert lines since late last year.
19 electric trains are now in use each day as part of the £800m South Wales Metro project.
Transport Secretary Ken Skates said: "The Class 756 state-of-the-art trains are transforming travel for passengers with faster and more frequent services as well as greater capacity, comfort, and improved accessibility."
"This is another significant milestone on our mission to upgrade the Core Valley Lines and deliver a turn-up-and go metro service."
TFW says the new trains will offer 50% more capacity, along with improved acceleration and more accessibility, including level boarding.
A trial was run earlier this year on the Coryton line, which will get a Sunday service for the first time from December.
But last March, TFW admitted that the realiability of services on the Coryton and Penarth lines had not been good enough following the introduction of a new timetable in June 2024.
Those changes also saw the new Class 231 trains removed from the Penarth line just over a year after they were introduced and moved to services covering Barry and Barry Island.
TFW claims that since then, at least nine out of ten trains on the Coryton line have arrived within three minutes of their scheduled time while the number of cancellations has been "significantly reduced".
Colin Lea, TFW's planning and performance director, said: "In June last year, we developed a new timetable to get the best out of our brand-new trains, which after a few tweaks was a success."
"Customers will now benefit from these changes with more capacity and reliability, level boarding and a better overall experience, as well as an hourly Sunday service from December.”
The next stage of the rollout, due by next year, will see the Class 756 trains introduced on the Rhymney and Vale of Glamorgan lines - with services from Rhymney to Barry Island and Bridgend.
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