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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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For those who have mobility problems the idea of a walk in the Welsh mountains may seem unlikely. But this new scheme may change that.
Any rain will eventually clear this afternoon, bringing brighter weather behind it.
Public Health Wales are warning visitors to be aware of the infectious diseases associated with livestock this bank holiday weekend