Advocates of greenways argue that an advantage of ripping up railway lines and converting them to greenways will allow for a railway to be put back at some future point as a greenway will allow preservation of the alignment for such a future use.
I would argue that once a greenway, a railway is not coming back as those that make use of the greenway, especially if they have developed a business around it, will absolutely oppose the closure of the greenway to allow rail based transport to resume.
This example from Edinburgh is a case in point. Proposals to extend the tram network in Edinburgh, which involve the restriction of use of a “travel corridor” by cyclists, are being opposed due to the aforementioned restriction.