After reading this thread (great thread, by the way!) I decided to go over to White Heath to try to find and examine more closely the marker post. I could not, however, locate the post among the 3-6 ft tall weeds and brush that were present. Not knowing whether it was on the N or S side of the segment did not help, either. I did make some observations, though...
Between Monticello and White Heath, it is very apparent that the PPOO (and 10) did not follow the Interurban. What might look like a road next to a track on some aerial views http://maps.live.com/default.aspx?v=2&...1&encType=1
is actually two tracks (Illinois Central parallel and north of Illinois Traction [interurban]). Eloise Ln. does parallel the tracks, but it is a newer road and is not very long. Interestingly, the interurban tracks are still present at the crossing near Eloise Ln. These are the first interurban rails I've ever seen.
I'm convinced that "1532 Rd" (old 47 on the USGS maps KtSotR posted) is the PPOO, and that at the point in question it is the road where the marker lies. I surmise that the PPOO then headed into White Heath until it made a forced Left turn onto S. Meridian. From there, it's hard for me to say how it got to the edge of town, but it did follow the tracks NE out of White Heath.
Anyway, that was my day. I sure wish I could have found that marker. I'll have to go back when the weeds are dead.
Wes