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Network Rail and Arriva Trains Wales say the new Loughor Viaduct will carry improved train services to West Wales.
It will carry two tracks between Swansea and Llanelli instead of the previous one.
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A charity match is to be held later tonight to raise funds for a former Cardiff Devils player with a terminal neurological condition.
Staying rather cloudy for much of the day in the south, perhaps with a little rain at first.
Mostly dry with clear spells developing tonight in the north, which will allow some mist and fog patches to form.