Infrastructure will need rebuilding on the West Coast Main Line after water levels reached higher than 2m on one section of the line.
Network Rail said a number of cabinets housing complex electrical equipment were submerged north of Carlisle following Storm Desmond.
Engineers have been working 24 hours a day to clear trees, landslips and repair flood damage but further works will be needed when the waters recede. There are currently no trains between Carlisle and Scotland.
Network Rail route managing director Martin Frobisher said: “The unprecedented amount of rainfall and high winds have taken a significant toll on the railway and we are working round the clock to keep as much of the network open and repair the damage that has been caused.
“North of Carlisle station we have to wait for 2.4m of water to recede before we can carry out extensive safety checks and repairs – including rebuilding a host of complex electrical and signalling equipment which is currently underwater. This, coupled with dozens of other sites which have been flooded or damaged, means we have a lot of work to do to ensure the railway is safe and able to be used.
“We care about passengers and will do everything we can to reopen the West Coast Main Line as quickly as possible. I thank passengers for their patience during this difficult time and can assure them we are working with the train operators to provide the best possible information. I urge anyone planning to travel in the North West in the coming days to check for the latest information before they set off.”