What does "Abandoned Railway Track" really mean?

The common view is that if no physical signs of the object (in this case, the railway right of way) remain then it doesn't belong in the database. For an abandoned track which has been completely obliterated by development (e.g. parking lots and buildings) this would lead to deleting the part where you have verified it no longer exists. If you have only verified that part of it no longer exists you should split the way and only delete the part that you have verified.

Keep in mind that the signs of a former railway can be subtle and often not seen from aerial imagery.

You can read up on the spirited history of railway=abandoned on the talk-us@ mailing list.

If a railway that has been dismantled is improperly tagged with "abandoned", then fix the tagging by setting it to railway=dismantled. Do not delete railways just because you can't see them. Maybe the evidence of the railway's existence is not visible to you.

Keep in mind that there's more to a railroad than the rails. With one example I'm familiar with, abandoned about half a century ago, much of the ten miles of former track has been turned into hiking trails, and the routing is blindingly obvious on the ground: the gravel bed is still there, as are the foundations for some of the signaling equipment. Even the areas that aren't trails, it's still fairly clear that something was there: bridge abutments, collapsed tunnel portals, parallel rows of trees lining an inexplicable clearing, a twenty-foot-deep chasm carved through solid rock, and so on.

OSM does not keep historic objects. This is part of the "on the ground" rule that say that a feature have to be on the ground for you to map it. You are safe to delete the dismantled railway tracks and potentialy send a message to the original contributor.

Although an abandoned railway leaves distinctive features which are not rails, most of these should be mapped for what they are: embankment, cutting, bridge, footway, cycleway.

For example, this bridge and the connecting paths used to be a railway (abandoned in 1970s) - nowadays, there's nothing at all on the ground that would warrant newly adding a railway=abandoned tag to the path. The bridge is there, as are the embankments, but nothing in particular says "railway" besides the distinctive embankment width http://www.openstreetmap.org/way/172953994#map=17/50.03605/14.46326

OTOH: this railway spur has moved down from active to abandoned, relatively recently (see also the shift from active to disused on the connected network) - it wouldn't make sense to remove the tag, even though it wasn't converted into anything: http://www.openstreetmap.org/way/27757285/history

So, IMHO, I wouldn't create a new way for an abandoned RR, but if an existing way gets converted into a cycleway, just switch the tag to rail=abandoned, as has happened here: http://www.openstreetmap.org/way/40759571

Some mappers love historic stuff http://wiki.openstreetmap.org/wiki/Historic so it shoild be left alone. If correctly tagged it probably won't render (or show) on the main maps but it can be seen when editing and used by history fans for their maps or research. Some dismantled lines are now cycle routes and long footpaths, I wish they all were.

In addition to all the points mentioned over the years there is one more addition that can be done here. I found this recently while working on railway networks. Most of the times the abandoned tracks also have the highway tags [highway=track or highway=cycleway] associated with them. In small towns and forest areas these abandoned tracks may still exist physically and in addition that path might have been converted as a cycle track or a walk-way for hikes. Hence even if OSM database stores the entities are physically present but also combine them with new usage scenarios. Example, Cycle Path #121539221 and Track #169329219.