Building an alternative to a major railway track destroyed in winter storms would be prohibitively expensive, the BBC understands.
The cost of alternatives to the coastal track at Dawlish in Devon are "eye-watering", said sources.
Network Rail is considering five options, including reinstating the Okehampton line.
The line, which links much of the South West with the rest of the UK, was destroyed by storms in February.
Network Rail is preparing a government commissioned report into the inland alternatives.
BBC South West Political Correspondent Martyn Oates said: "It would appear to leave strengthening the existing line, a few feet from the sea at the bottom of the cliffs, as the only option.
"However, it's understood this itself has yet to be costed."
Network Rail said in a briefing to MPs in February that reinstating the Okehampton line would cost up to £700m.
Network Rail's options are:
- Reinstate the Okehampton line (between Plymouth and Exeter, via Okehampton), which closed in 1967
- Create a new line connecting existing freight lines from Alphington (near Exeter) and Heathfield (near Newton Abbot)
- Three options between Newton Abbot and Exeter via Teignmouth (with new tunnels)
The briefing also suggested making the existing coastal railway "more resilient".