Almost half the length of the main rail lines in the South West are at risk of flooding because of climate change, an internal Network Rail report claims.
The study released to the BBC found 42% of primary rail routes could be disrupted.
Entitled Increasing Climate Change Resilience, it said "extreme weather events" are likely to be more frequent.
The report said Network Rail is developing a "climate change adaptation strategy" worth millions of pounds.
The report classed "extreme weather events" as the storm which breached the sea wall at Dawlish, Devon in February, causing weeks of rail disruption.
The report listed particularly vulnerable areas as:
- The seafront stretch of rail line between the Exe Estuary and Newton Abbot, which includes the Dawlish section
- The Somerset Levels, where large areas were underwater for weeks earlier this year
- The Cowley Bridge junction, outside of Exeter, which has suffered repeated flooding
Disruption to rail services into and out of Devon and Cornwall can cost the region's economy tens of millions of pounds, the study notes.
The chairman of the South West Peninsula Rail Task Force, Councillor Andrew Leadbetter, said: "I didn't realise just how vulnerable the main rail lines were.
"I am concerned. This means we have to lobby the government even harder to make sure our vital train links are protected."
Mike Gallop, Network Rail's director of route asset management, said: "What we're doing is developing a strategy to look at some of the possible solutions and mitigations and then working over the next 15 to 20 years to make sure we can hold the railway line to the south west peninsula."
The government is carrying out a feasibility study into an alternative rail line, which would run inland from Exeter to Plymouth, via Okehampton, avoiding the vulnerable stretch around Dawlish.