The news comes amid continuing concern about delays to reopening part of the Borders route, with a date for the start of construction yet to be set.
The Edinburgh-Carlisle line became one of the most controversial casualties of the Beeching cuts in 1969, with protesters blocking the last train.
Waverley Route, by David Spaven, claims that the absence of two supporters of the line at a ministerial committee meeting in 1968 that decided to close it “may well have been crucial”.
They were Board of Trade President Anthony Crosland and his junior minister, Lord Brown of Machrihanish, whom Scottish Secretary Willie Ross had counted on to back his opposition to closure.
Mr Ross also failed in a final plea to Prime Minister Harold Wilson, despite warning that then MP David Steel had threatened to stand down and force a by-election over the issue.
The book further reveals that transport minister Richard Marsh, who implemented the closure, later privately admitted it had been his biggest mistake.
Mr Spaven, a rail freight consultant and member of the Campaign for Borders Rail, described the project to reopen a 31-mile section of the route between Edinburgh and Tweedbank, south of Galashiels, as “one of the most remarkable rail projects in modern British history”.
However, he said campaigners were right to be worried about rumours that costs could be cut by reducing the proportion of double track, increasing end-to-end journeys from 55 minutes to “an hour or more”.
Transport Scotland said this was “misleading and inaccurate”.
Ministers have also insisted the line will be opened by the end of 2014 and within its £295 million budget, despite the start of construction now running nearly a year late.
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