The West Highland Line will not reopen until a further two weeks following assessments from Network Rail engineers.
It was hoped train services would be reintroduced between Crianlarich and Oban from Monday, but the line will instead remain closed between Ardlui and Crianlarich until Thursday August 22.
Detailed geotechnical and aerial surveys of the damage between Ardlui and Crianlarich have now been completed, allowing Network Rail engineers to fully understand the extent of the repairs needed to reopen the railway.
The disruption began when heavy rainfall caused the slope supporting the railway to give way at several points on a section of track in Glen Falloch on Sunday.
Floodwater washed the track-bed out from beneath the line and engineers will have to replace thousands of tonnes of lost material to rebuild the supporting slope before they can repair the track itself.
The flooding also damaged ballast under the track on the Oban branch line, near Tyndrum.
The recovery works are further complicated by the remote location of the sites and the ongoing weather conditions.
To keep passengers moving ScotRail services are currently running between Glasgow Queen Street and Arrochar/ Ardlui and between Crianlarich and Fort William / Mallaig. Replacement buses are operating between Arrochar and Crianlarich, and to Oban.
The Caledonian Sleeper has been forced to cancel some services from Fort William to London Euston. Departures are now taking place from Edinburgh Waverly until the line is fixed.
Liam Sumpter, Route Director for Network Rail Scotland, said: “Our engineers will be working around-the-clock to reopen the line as quickly as possible for our customers.
“This is a very complex repair as we need to reconstruct the slope beneath the line – replacing thousands of tonnes of lost material – before we can repair the tracks above.
“We are keeping passengers on the move. ScotRail trains are running between Glasgow and Arrochar and Ardlui, and between Crianlarich and Mallaig. We also plan to reopen the line between Crianlarich and Oban on Monday.”
A final timetable will be confirmed in the coming days and customers are advised to visit the ScotRail website for the latest information.