Passenger numbers are falling on the route between Inverness, Wick and Thurso, which has been plagued by chronic unreliability and frequent cancellations.
In Highland Survivor: the story of the Far North Line, rail consultant David Spaven argues for urgent action to reverse its fortunes.
The 168-mile route was the biggest reprieve among those earmarked for closure by the Beeching report 50 years ago.
However, unlike most Scottish lines which have enjoyed booming traffic, Britain’s longest rural route is declining and many journeys are far quicker by road.
Spaven wrote: “Closing the line – in the short to medium term – is politically unthinkable.
“However, this is not the same as saying the Far North Line will be impregnable in the longer term: once the current [ScotRail] franchise ends [in 2022/25] and/or after nuclear traffic finishes and/or in the event of future economic recession, or even depression.”
Spaven estimated the line lost up to £20 million a year - or £40 per passenger.
He said: “Bus campaigners could reasonably argue a tiny fraction of the financial support needed to maintain the line would deliver state-of-the art road-based public transport.”
The Friends of the Far North Line called for significant extra spending, including on more passing loops, to “reverse this shameful decline in standards”.
Convenor Mike Lunan said: “The service has deteriorated hugely in the last 15 years.
“Trains are timetabled to take over 30 minutes longer than in 2001, and yet less than half of them manage to arrive on time. Caithness has been appallingly let down.”
A ScotRail Alliance spokeswoman said: “We have no plans to close the Far North line and have recently invested £6m in improving infrastructure along the route.
“We are working hard to deliver improved performance and are committed to providing as punctual and reliable a service as we can.”