Difficult to apply retrospectively, that one: too many of the trackbeds simply no longer exist, having been built on, or used as landfill tip (a favourite re-use option for disused railway cuttings!). Since, hopefully, the main spate of Beeching-axe closures which had its peak in the 1960s, has since abated, there won't be that many newly-closed railways to take on board for this scheme.
The one advantage, from some points of view, of disused railway tracks, is that they are fairly level for folks who don't like hills. But the disadvantage, for me, with existing ones that I know of, is the poor surface, quite unsuitable for road bikes.
Furthermore we have organisations like the Bluebell Railway working hard to reinstate trackbeds as working railways. They are currently working on an northbound extension of their existing line, linking up to East Grinstead. Which means they have to empty out a landfill dump. Best of luck to them, I say!
Perhaps we should be content with what we already have.