Nick: The tunnel location puzzled me too. Turns out there were actually two tunnels: one at each end of the huge washout below Lincoln Park where there's now a long staircase and diversionary trail. I've marked their locations on a 1910 map uploaded to my web gallery at
http://tinyurl.com/den4t5The east tunnel was removed first, probably by 1895, and the east tunnel survived until 1905 when the right-of-way was widened to make way for streetcar operations.