Any area on Nimbus not covered by a polygon generally means this is unregistered land. Land registration only became compulsory in the 80's/90's so any properties/land parcels which haven't changed hands since then (as will be the case for many railway lines) will not show a polygon and will not have a title, so we wouldn't be able to confirm ownership. (note: the unregistered land pin only appears where we have a URPN, so you won't see the unregistered pin appear on open land or other similar parcels of land (like railways) that don't have any UPRN associated with them).
These sites are usually owned and managed by Network Rail, so I would suggest reaching out to them to continue with your investigation