The £25m refurbishment of Barmouth Viaduct is underway and is expected to take three years to complete.
Network Rail will be replacing large amounts of timber and metal parts of the Grade II listed bridge, as well as the entire length of track.
The upgrade will mean the railway and footbridge are closed at certain points, causing some disruption to travellers who would normally use the route. The first closure starts on October 16.
Network Rail have said they will adapt their plans when necessary to ensure their work is inline with the latest government guidance regarding Covid-19.
Bill Kelly, Network Rail’s route director for Wales and Borders, said they are "confident" they will be able to carry out the work safely "and in line with social distancing guidelines for construction work.”
The viaduct is more than 150 years old and serves trains on the Cambrian Coast Line.
Work on the historic landmark will take place in three stages. The bridge, including the footpath, will be closed for periods of time with the first closure taking place from 11:40pm on Friday 16 October to 05:30am on Monday 2 November.
Next year, the viaduct will close from 17 September to 18 December. The dates for 2022 are not yet confirmed but are likely to be between October and December.
Mr Kelly said the £25m restoration is the biggest in the bridge's history and will "mean we can make sure Barmouth Viaduct can continue to serve people for many years to come".
Transport for Wales' CEO, James Price, said he was "delighted" that the work was taking place.
He added that they are "working collaboratively to ensure minimal disruption for our passengers while work takes place" but passengers who usually use the Cambrian Line should check for timetable changes before they travel.
Gwynedd Councillor Eryl Jones-Williams, the chair of the Cambrian Coast railway forum, said: "I am pleased Network Rail has taken into account how important Barmouth Viaduct is to the local community and tourists by trying to minimise disruption as much as possible.
"The upgrade will help maintain connections between the communities to the north and south of the Mawddach estuary."