As far back as 1975 or so, the MC line from Kzoo to Porter has always had that “tan” color ballast. At one time the entire MC mainline all the way to Detroit had that ballast. Given the yellow tint, I’d guess somewhere outside the great lakes limestone belt. Perhaps somewhere along the east coast. New York State? That might explain the similarities between the NYC era ballast and the Amtrak era ballast. ( Conrail ballast was dark/medium gray).
Another possibility is that Amtrak didn’t replace the ballast, but rather had it “cleaned” by a contractor such as PANDROL or SRS. ( GTW lines in Michigan had similar ballast in the 1970’s). It all boils down to the cheapest supplier at the time, some secondary lines used cinders for ballast which was the cheapest of all in the age of steam.
CSX has been getting granite ballast from the Lake Superior region via the Toledo Docks for years. It’s more durable than the sedimentary based rock native to Ohio. I’m not sure if NS still gets ballast out of Findlay or not.