A flood-damaged section of the rail link between Glasgow and Edinburgh could take two months to repair, Network Rail has said.
The Union Canal breached at Polmont near Falkirk after heavy rain on 12 August, damaging a 300m (984ft) section of track.
Network Rail said thousands of gallons of water had washed away parts of the track and undermined the embankment.
Engineers were able to fix the canal breach and stop the flooding on Friday.
However, more than 1km (0.6 miles) of track and ballast will need to be replaced.
A 30m (98ft) section of the Union Canal at Muiravonside breached last week, flooding the railway between Polmont and Bo'ness on the main Edinburgh to Glasgow route via Falkirk High.
Network Rail said it would be working "around the clock" to repair the flood damage from "torrents of water" which cascaded down from the canal above.
The power of the floodwater undermined and washed away overhead electrification masts and damaged others. Dozens of uprooted trees have also been strewn across the tracks.
Kevin McClelland, Network Rail route delivery director, said: "We are working with our asset engineers and specialist contractors to assess the scale and extent of the damage and what we will need to do to safely reinstate the railway.
"It is remarkable to see the destructive power of the flowing water and the extent of the flooding and the scale of the damage is something I have never witnessed before on the railway.
"We are grateful to our colleagues at Scottish Canals for their prompt response in dealing with this unprecedented incident."
Passengers who wish to travel between Edinburgh and Glasgow or Dunblane should contact ScotRail for details on alternative routes and bus replacements.