Alternative rail lines to the storm-hit coastal route through Dawlish in Devon offer "poor value for money", according to a new Network Rail report.
The government commissioned the report after fierce winter weather caused the line to collapse, cutting off mainline services to Devon and Cornwall.
Seven alternatives were considered, as well as the continuing maintenance and strengthening of the existing route.
Network Rail said the new route options were "unpromising".
The report estimates a continuing maintenance regime on the current line could cost between £398m and £659m over 20 years.
Alternative options were:
- Route A - the former London and South Western Railway route from Exeter to Plymouth via Okehampton;
- Route B - constructing a modern double track railway on the alignment of the former Teign Valley branch line from Exeter to Newton Abbot;
- Five alternative route Cs (C1 - C5) - providing a new line between Exeter and Newton Abbot.
National Rail appraised each route in line with Department for Transport guidelines, where the project benefits and costs ratio (BCR) measures the net economic benefits per pound.
Schemes with a BCR of greater than 4.0 - £4 of benefit for every £1 spent - are deemed to be of very high value for money, while schemes with BCR of less than 1.0 are considered poor value.
It found Route A offered a BCR of 0.14, with route B at 0.29 and the C1-C5 alternatives between 0.08 and 0.17.
Transport Secretary Patrick McLoughlin said: "This study is an important step towards achieving that goal and providing the region with a rail network that helps it thrive.
"I will now consider its contents before making an announcement on next steps later this year."
South West Devon MP Gary Streeter told the BBC he remained confident the government would settle on one of the various alternative new routes, rather than simply relying on shoring up the existing line at Dawlish.
Mr Streeter said the report was "only a step".