The proposals have been submitted to Carmarthenshire Council to replace the Loughor viaduct and install an additional six-mile track between Cockett West Junction and Duffryn West Junction.
The disused eastbound platform at Gowerton station will be reinstated to cater for the new track.
Network Rail hope to start construction by next Spring with work scheduled to last at least 12 months.
Mark Langman, route managing director for Network Rail Wales, said: “Today’s announcement of our plan is another affirmation of Network Rail’s commitment to growing the railway in Wales.
“We hope that local authorities, passengers and the local community share our vision of the long term benefits and support us during the construction works to deliver the scheme successfully.”