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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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UKIP Assembly Member Michelle Brown has apologised after a recording emerged of her using a racial slur.
There will be a number of road closures in place in Cardiff today as the British Speedway Grand Prix returns to the Welsh Capital.
Today will be a day of often heavy showers, with the risk of hail and thunder.