Work is due to start next month on a £20m project to replace the wooden Pont Briwet viaduct near Porthmadog in North Wales with a new bridge.
The contract has been awarded to Hochtief(UK) Construction with completion set for February 2015.
The current 22-span structure carries the single track Cambrian Coast railway from Dovey Junction to Pwllheli and a single track toll road across the Afon Dwyryd for cars.
Heavier and larger vehicles, such as lorries and buses, are unable to use the existing bridge and face an extra eight-mile journey from Penrhydeudraeth to the Harlech road.
The proposed new structure will continue to carry the single track railway but also provide a two-lane public highway and cycle route and the current toll fees will be removed when the project is complete.
Councillor Barry Thomas, chair of the Mid Wales Transport Consortium Board, said: “Discussions relating to Pont Briwet Bridge have been ongoing for some time and I am delighted that it can now finally proceed.
"This will be a major boost for the area, and will undoubtedly have positive impacts for rail and bus passengers with shorter journey times as well as drivers of larger or heavier vehicles who will be able to take advantage of a far more direct route across the river."
The new bridge will be 18 metres wide compared to the existing 8.5-metre wide structure. It will be built in phases so that train services will not be unnecessarily disrupted. A temporary bridge will also be provided for vehicle crossings.