Dow has the main plant in Midland (traffic answered in another post) and another plant at Ludington, which is on the aforementioned Marquette line. The last mile of the Marquette line in GR is controlled by CSX, but on their website they say they also interchange with the Grand Elk. Speaking of the Gand Elk (former PC Grand Rapids-Elkhart) a source of auto company traffic on the line was the former Fisher Body plant just south of Kalamazoo. The plant closed in 1999 but has since become an industrial/logistics park that still has rail service.
https://detroit.cbslocal.com/2010/08/24/life-after-gm-abandoned-kalamazoo-auto-plant-humming-again/
For present traffic on the Grand Elk see:
http://railroadfan.com/wiki/index.php/Grand_Elk
Back in the early 70s I used to live in K’zoo and would see lots of 85’ hi-cubes in the area. I got the impression that some of the Detroit-Elkhart overflow traffic that did not use the &