Network Rail wants to replace the brick parapets on both sides of the Grade II Listed structure which runs beneath St Paul's Road in Deepdale.
But the project will mean traffic, including regular bus services, will not be able to use the busy road while work is going on.
The bridge, over a branch line of the former Longridge Railway, will need to be demolished down to road level and rebuilt.
Years in pictures: Here's 37 pictures from 2009 showing what the people of Preston were getting up to
Picture past: Here's 29 pictures going back to 2000 and showing just what life was like in Preston
Long lost Preston pubs: Here's 33 pictures of forgotten boozers in Preston that no longer serve pints
School's out: Here's 36 scenes showing from what life was like at Archbishop Temple High School in the 90s and 00s
How Olivia Newton John inspired these Preston and Chorley Grease productions from years gone by
Because the brickwork, much of it original, is now degraded after almost two centuries of weathering and pollution, Network Rail says it can no longer be patched up. And unless the walls are replaced they could become unsafe.
"There is no other action available other than to replace these bricks," says the company.
"Following a detailed examination of the bridge it has become apparent that the majority of the brickwork to the parapets has now become degraded through age/weathering/pollution etc.
"This has caused the surface to blow. This in time will lead to quicker deterioration of the old brickwork with the loss of integrity to the parapet wall.
"The interventions proposed are all intended to secure the longer-term structural integrity of the structure, whilst also ensuring that the parapets remain safe for member of the public."
Specialists are to source new hand-made bricks to match as closely as possible the ones being replaced. They will also be using lime-based mortar because the bridge is a Listed structure.
The heritage asset was first constructed in 1846 which is in the "Heroic Age" of railway building and it is deemed to have "strong historic significance."
It was part of a line between Deepdale and the Maudland area of Preston to link the Longridge Railway with the Fleetwood, Preston and West Riding Railway.
Network Rail is planning to work seven days a week for six weeks in an attempt to get the project finished as quickly as possible.