Dunragit is to get a replacement railway bridge after severe corrosion was discovered on the underside of the old Challoch Bridge.
The bridge, which until this summer was the most-struck bridge by lorries in the UK, carries the single track Glasgow Central to Stranraer line over the road.
With the relocation of the A75 to the new bypass nearby earlier this year, works to investigate the bridge revealed the issues.
Network Rail said: “The existing bridge has been surveyed and the results indicate that there is severe corrosion.
“This would have resulted in extensive repairs to the deck. In the interests of value for money it has been decided to replace the bridge with one which will have a design life of 120 years.”
A masonry condition survey of the abutments and wing walls was also undertaken during the site inspection and Network Rail said this would have to be addressed.
The company said: “In general the substructure was found to be in a poor condition as a consequence of multiple vehicle strikes, particularly to the quoins. Repairs, including localised stitching, grouting repairs and strengthening, will need to be conducted prior to installation of the new bridge deck.”
Planning permission is being sought from Dumfries and Galloway Council before any works will commence.