Some inconsistencies in the photos I linked to above and the OP photo led me to explore some more. I knew Belleville had at least two mills on the eastern hillside, but didn't know that BOTH had curved retaining walls with RR tracks. The one where the RR approached on a trestle is the one farther east, not the one I linked to in my Google aerial link above. Here is the correct Google aerial, in which you can see the scale pit Brian mentions, and the earth footings for the trestle next to the retailing wall.
Google aerial link to the mill with the pit and trestle
Bruce