Large railroads (called "Class I" railroads by the Surface Transportation Board, those having more than $255 million in annual revenue) are required to annually furnish maps on which they indicate "lines subject to abandonment within five years", lines for sale, and other categories. These maps are available to the public.
STB rules require that before an abandonment takes place, the line should be offered for sale to any "financially viable entity". If none appears, the abandonment goes through.
Check out
www.stb.gov
That's probably the best single place to go for abandonment info. You can also visit the American Shortline and Regional Railroad Association Web site. Do a search; I don't know the URL offhand.
Recently railroads have been leasing marginal lines to short lines rather than selling them, for a variety of reasons. A lease preserves the Class I's ability to control traffic, and also avoids certain legal liabilities. But this is a complex subject. Go have a look at the STB site.