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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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The Plaid leader urged voters to give her party a mandate, so that she can give Wales a voice in Parliament as the SNP has for Scotland.
90,000 people are expected to attend the youth festival, which runs from the 29th May to the 3rd June.
Heavy, thundery showers will clear northwards through the morning, allowing it to turn mostly dry and bright for many.