A Grade II* listed railway viaduct will undergo a £25m renovation dubbed “the biggest upgrade in its history”.
Barmouth Viaduct in Gwynedd will have a large number of the timber and metal elements replaced and a new track installed.
The viaduct provides a link between Machynlleth and Pwllheli on the Cambrian Coast Line.
Network Rail said it had worked with heritage body Cadw and Gwynedd council on the planned redevelopment.
Bill Kelly, Network Rail’s route director for Wales and Borders, said: “Barmouth Viaduct is one of the most celebrated and recognisable structures in Wales and is the only major timber-built bridge still in use.
“We are investing £25m to give Barmouth Viaduct the biggest upgrade in its history, protecting our industrial heritage and ensuring this vital transport link can continue to serve local people and visitors, when the time comes, for generations to come.
James Price, chief executive of Transport for Wales, added: “As our services will be affected, it’s important for anyone travelling on the Cambrian Line to check before you travel, in case of any changes to our timetables.