Network Rail video
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.
More top news
Not the brightest of days today! Showers will spread from west to east, with some sunshine possible later!
Tyson Manchester United Keane, 31, became abusive while trying to update the name on his driving licence.
Donna Williams, 33, says she was dismissed from her NHS clerical job of 15 years and made to feel 'victimised' for wanting a family.