A £40m plan to boost rail services in West Wales has been given the go-ahead with permission granted to replace a bridge on the main route between Swansea and Llanelli.

The plan will see the redoubling of six miles of track between Cockett West Junction and Duffryn West Junction, the re-instatement of the disused platform at Gowerton station, and the replacement of the Loughor viaduct.

Funding is split between Network Rail, which is paying for the £16m viaduct works, and the Welsh Assembly Government, which is behind the track redoubling.

The grade two listed Loughor viaduct has to be replaced as it can only bear the weight of one train at a time.

Listed building consent for work on the structure was received from Carmarthenshire Council on May 31 and preparatory work will continue on site with the intention of opening a new bridge in mid-2013.

Network Rail is going to preserve some elements of the original viaduct and will rebuild two of the original spans on railway land to the west of the existing structure. It will also carry out a full photographic survey, so a record of the viaduct is available for future reference.

Route managing director for Network Rail Mark Langman said: “Replacing the Loughor Viaduct is the key to improving rail services to West Wales and we’re keen to get on with the work on this vital link.

“We appreciate the need to preserve our industrial heritage, we also have to provide the people of Wales with the best railway possible. In this instance we are working hard to keep some of the structure in place, whilst also taking care of the natural environment of the Loughor Estuary.

“The improvements to the railway between Llanelli and Swansea won’t just allow for more trains, they will also make the existing services more reliable and give the economy of the area a welcome boost.”

As the Loughor Estuary is a site of special scientific interest, the work has been planned in close communication with the Environment Agency for Wales and the Countryside Council for Wales to minimise any impact on the environment.

Two platforms will be installed on either side of the bridge to effectively turn the structure into a quayside. This will then allow a barge to be floated up the estuary to tie up next to the bridge. Another barge will be lowered in from the other side of the bridge. These will then sink pilings into the bed of the estuary so construction can continue around the operating railway.

All spoil will then be taken way, rather than pumped back into the water, protecting wildlife.

Thanks to the way the new structure will be built alongside the existing viaduct, the railway will only have to be closed for an estimated three weeks – planned for Easter 2013.

By that stage the foundations for the new structure will be in place in the estuary, with the replacement bridge deck next to the original, ready to be slid into place.