Under a £16m contract Carillion will design and build a viaduct capable of carrying two tracks to replace the existing structure on the current alignment.
The job forms part of a Network Rail and Welsh Government jointly funded £40m plan to improve rail infrastructure along the route, which will also see 6 miles of new track built between Cockett West Junction and Duffryn West Junction.
Passengers levels using railways in West Wales are forecast to rise by 20% by 2020. The plans will meet that demand, allowing two extra trains per hour between Swansea and Llanelli and one extra an hour between Swansea and Carmarthen.
An extra line should allow trains to carry on running when one of the lines is shut or congested.
Gowerton station will also be upgraded with a new footbridge to allow access to the new platform, CCTV, real-time customer information and better lighting.
The scheme promises to bring huge social economic benefits in southwest Wales by improving capacity and connectivity to key employment centres such as Swansea, Llanelli, Carmarthen, Fishguard and Milford Haven.