New electric 'tri-mode' trains are being rolled out for the first time onto the South Wales Metro network.
Transport for Wales says the Class 756 trains - which can also use diesel or battery power on non-electrified routes - are being introduced on the Merthyr and Aberdare lines first, followed by the Treherbert line.
But passengers in the eastern Vale of Glamorgan will have to wait until late next year before the new fleet is brought into service locally.
TfW says the phased introduction will see fourteen 'tri-mode' trains introduced by next Spring on the electrified Valley lines - the first service of its kind in the UK.
James Price, TFW's chief executive, said: "Our teams and delivery partners have been working on transforming our rail infrastructure in South Wales for the last three years and this signifies the next phase of the South Wales Metro."
"We’ve been modernising and electrifying the railway line and we are now extremely proud to be able to introduce the first of our brand-new trains that will be powered by electricity."
"We’ve already introduced sixty-five brand-new trains to the whole of our network and we’re excited to now add our Class 756 electric trains that will offer customers more capacity, improved seating, modern air conditioning, power sockets, WiFi and passenger information screens with up-to-the-minute travel information. They’ll also feature automatic level boarding and space for up to six bikes."
From late 2025, the Class 756 trains will start moving over to the Rhymney and Vale of Glamorgan lines, when the new tram-trains enter passenger service on the Valley lines.
From then on, the 756s will run from Rhymney to Barry Island and Bridgend and from Coryton and Caerphilly to Penarth.
In June, major timetable changes saw 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.
More recently, services to and from Penarth have also been affected by engineering works as part of the Metro project.
But TFW says the transformation of the Core Valley Lines will enable faster and more frequent services between Cardiff and the heads of the valleys.
Emil Hansen, commercial project manager for Stadler - which built the new trains - added: "We are thrilled to see the UK’s first FLIRT tri-mode enter passenger service in Wales."
"Using battery technology, these transformative rail vehicles represent a significant leap forward in Transport for Wales’ efforts to drive decarbonisation and underscore Stadler’s trail-blazing technology and commitment to clean energy."
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