Abandoned for rail use though. If you are hiking somewhere like the allegeny national forest, these abandoned rail routes are a boon since they are generally raised up out of mucky ground, flat, and easy to navigate along if you know which one you are on. Hunters frequently just walk along the abandoned railways when searching for game. Why not call it a trail at that point? IMO because this isn't built like a trail (which can have complicated terrain or scrambles up or down some steep terrain), its built like a railway, and should be marked as such on a map since that will tell you a lot about the conditions on that route (raised, flat, generally free of obstructions short of fallen trees since abandonment)
Usually rail trails are deliberately constructed and maintained however. Although the degree to which this is done varies. And it's also not uncommon for some section of a rail trail to have been worked on and other sections are in the plans for someday--whether because of money or local opposition.
ADDED: And as noted elsewhere the railroad or its successors in ownership often still own the right-of-way--however unlikely it would be to revert a popular rail trail to rail use.
