Dave,
I see a faint trace that may be the wye that your talking about. You may be right about a construction wye.
A couple of years ago while passing Belleville I stopped and walked around and I think I found the wye that once existed there to the northwest. It shows up in a UNR Special Collections photograph that is an overall view of Belleville looking down from above the upper mill that was published is Farrell's book. All that I could find there is slight grading and maybe some other disturbance of the surface. There was no sign of ties, etc. That matches the description of your site.
For the on-line digital version of the above mentioned photograph use this link:
Belleville
Brian Norden