A £48m project to replace the Loughor Viaduct and upgrade the rail infrastructure between Swansea and Gowerton is one of the improvement schemes currently underway.
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The rain may become increasingly patchy later in the afternoon, perhaps allowing some drier and brighter interludes.
Cloudy skies overnight, accompanied by patchy rain through the first part of the night.
Here are all the details you need to know before heading into Cardiff to watch Justin Bieber on Friday.