This new viaduct and redoubling of the railway through Gowerton will remove the current bottleneck created by the stretch of single line and provide capacity to run more trains in the future.
This project joins a number of other rail improvement projects in South and West Wales that will help deliver better journeys and boost economic growth in the area.
As well as making it easier to travel to and from West Wales, they will help to boost local employment opportunities.
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The three-day event in the Welsh capital is expected to attract more than 20,000 visitors.