The Guardian carried an article on 31st August 2019 about old rail routes being used as cycleways. It suggested the 10 best routes where old railway formations are in use as cycleways. Theirs is not the only list of routes which seeks to provide a "Top Ten."

I have pulled together a few examples in the linked post below. I'd like to add at least one which does not feature in the top ten lists, and that is the Forest of Dean.

http://rogerfarnworth.com/2019/09/13/railways-and-cycleways-no-1-a-top-ten