I know a fair amount about this line as I grew up on a dairy farm just west of Mosinee, WI during the years that this line was still in service and used to get to Marathon City (just west of Wausau) with my family on a regular basis, and once in a while to Edgar, just west of Marathon. I remember seeing the rail still being in although the only time I ever remember actually seeing a train on this line was as a very small boy (I was on an outing with my head start class to the zoo in Marshfield-one of the key attractions of that day was the old Soo Line steamer that still sits outside the zoo) at Stratford, just north of Marshfield. My dad worked for the former Marco Co-op at Halder, WI and had to make frequent trips up to Marathon to pick up feed, fertilizer, etc. from off the rail cars. Also a friend of mine remembers the line running through her uncle's farm between Marathon and Wausau, the bridge over the Rib River at this point is still intact. The thing I remember most about this line were the wig-wag signals at both Marathon and Edgar. One other recollection-my brother-in-law's late father worked for the C&NW as a switchman out of Eland and remembered working this line all the way to Marshfield. He said that he never worked west of Marshfield as that was the Omaha Division.
Just a couple of corrections, the town names are "MERRILLAN" and "WAUSAU". There is a good history of the line between Merrillan and Marshfield, which was part of the Omaha Western Division in Northwestern Lines which you could find back copies of on the C&NW Historical Society site.
I was able to trace this route after looking at Topographical maps from the 40's. A link to the route, as well as many of my other traces is here - just look up Merrilen to Wausau