SomeoneElse
Hello and welcome to OSM!
Looking at the data, "an old railway" is exactly what it is tagged as in OSM. What seems to have happened is that there's a proposal to run a cycle route over it (see https://www.openstreetmap.org/relation/3988061 ) but I'm guessing that whatever map you're using doesn't understand the "proposed" tagging on there (which is confusing, at least to me).
Best Regards,
Andy