The line will remain closed between Ardlui and Crianlarich and between Crianlarich and Oban until at least Monday, August 12.
Engineers will be working to repair track at sites around Ardlui, Crianlarich and Tyndrum where heavy rain and flooding has washed the track-bed and ballast out from under the railway.
At the worst affected sites, between Ardlui and Crianlarich, more than 1,000 tons of material has been displaced.
To keep passengers moving ScotRail services will continue to run between Glasgow Queen Street and Arrochar/ Ardlui and Between Crianlarich and Mallaig. ScotRail is also working to provide replacement buses.
A Network Rail spokesperson said: “Our engineers are working hard to fully assess the repairs required to reopen the line as quickly as possible for customers.
“The flooding over the weekend has caused significant damage at multiple locations along the line and we will need to rebuild the track-bed and supporting embankments in some locations before we can reopen the line.”