(1) The mothballed CRIP/Missouri Central StL-KC Line
(2) The Mothballed or partially abandoned Peoria - Champaign Peoria & Eastern Line either side of Mansfield IL
(3) The abandoned Dawson RR or ATSF/DC&CV Raton Pass Bypass (Boise City-Felt-Clayton + Dora-Farley-Colmor)
(4) The UP/Kansas Pacific/Arkansas Valley RR Kit Carson-Las Animas line
(5) The M&O/GM&O Mobile to MS State Line segment
(6) The D&NO/C&S Denver to Pueblo Line + Colorado Springs Branch (or CRIP Falcon-Co Springs)
I wouldn't include the CRIP StL-KC line. It hasn't been used for a long time. Even SP, after it acquired it, never used it, opting instead for trackage rights over the UP (ex MP) route between the two cities Since it remained in place (unlike some of the other candidates) it could have been restored relatively easily, if there were a reason to do so. There hasn't been, and it's unlikely there ever will be.
Last year, Missouri Central abandoned a segment on the western end of the line that severed the route (STB Docket No. AB-1068X), The ROW will probably be made into a trail under the National Trails System Act, so the ROW is technically available for reconstruction of the rail line. But, as a practical matter, the abandonment seals the fate of the line as a through route..