Railways have been under the threat of closure for decades.
Only here in west Sweden, there are at least three railways which are subject to close during the next decade.
Is it of interest to map them, while the embankment is still there?
Of course, you first need to ensure there are no trains that may approach. 
Has anyone tried to map on a rail trolley, or from the inside of the train driver’s cabin?
Here in the US the right of way is private. If it’s an active railroad, it’s not safe.
I did map some old tracks here in Knoxville. They’re not abandoned but dormant. I do urge people to be mindful that they could be wrong, though.
http://www.mapillary.com/map/im/qBSwifCOD4R8Tbsck0BnXA/photo
Whatever the case, it’s slow going to walk the tracks. I didn’t think of it before but it would be good to reach out railfans. They may be able to work with the local railroad, here in the US we have a loto of lightly used tourist lines, and get some pictures from the cab.
I’m sure it’s interesting to map them! For instance, my parents tell me there used to be a railroad where I grew up (in Estonia) which is really hard for me to imagine - would be cool to see, wish Mapillary had been around 30 years ago. 
Also it may be of some professional use some day, you never know!
There are a number of railway sequences on Mapillary, also done from the driver’s cabin - e.g.
http://www.mapillary.com/map/im/P-Eb9wf1hd4kqb8W4eYNvA/photo http://www.mapillary.com/map/im/RWOPl88vAwZAiVV8f2iOGw/photo http://www.mapillary.com/map/im/s7sI6l_ojZykkJEy9oIPYQ/photo
If you can find information on the path the old railbed took, you can always try to add it to http://www.openhistoricalmap.org/. I think the people who started that up are big rail fans, so they’d probably enjoy the help.