Initially the west half of the old main was cut back to the west bank of the Flint River. Sometime soon after 1985 that was cut back to Grand Traverse. Then Ballenger, then Linden Rd. in the 2000s. Delphi West Plant 10, the last operation GTW served at the former "Chevy in the Hole," stopped using rail service by Aug. 2000 and apparently transloaded at Corunna Yard for a brief time after that.
The east half was abandoned from downtown in 1974 or 1975 to just east of Longway. The Harrison St. freight house had GTW offices in it right up to abandonment and was replaced by the Bristol yard offices. I checked the MDOT right of way maps online. Longway west to end of track was all bought by the state from GTW in May 1975. This was done to acquire all the GTW right of way necessary for I-475 and UM-Flint. In Jan. 1976 the state sold to the City of Flint the ex-GTW parcels needed for UM-Flint.
The old main was cut back to Franklin by Jan. 1976, when the GTW track diagram for that area was redrawn, copies available through GTWHS.
From Dort to Franklin was removed in 1987 or 1988. The FRA database records Dort crossing closed by 4/19/88. The track was kept in place to Dort to allow the GTW to reach AC spurs that were accessed by the sharply curving NW quadrant wye, though I don't think it was used for that purpose much by then, if at all. Maybe once a year I'd see a few covered hoppers parked west of the diamond. Once I saw the GTW emergency crew training tank car parked there. Dwarf signals remained active on either side of the old main to protect the diamond. From just east of Dort to Averill Ave. came out circa 1998. Memory is hazy but it's possible that for very briefly the track from Averill to the diamond stayed in place, after the diamond to Dort was removed.
Adding to what Elba Steve mentioned, I checked the FRA database and saw that Walnut and Decker had mast flashers. There's no listing for any crossing from Longway west to Harrison. The FRA crossing inventory program appears to have started about 1975, when that stretch was abandoned. GTW had to stop and flag all movements over Vernon, as noted in employee timetables.
This site has info on old customers. http://gtwflintsub.blogspot.com/p/blog-page_8127.html McCloy Paper & Stationery was the last customer west of Dort and got boxcars until 1983 or 1984. It remained open for some years after that with truck service only.