This major branch of the Pennsylvania Railroad left the mainline on the West Branch of the Susquehanna River at Elmira Branch Junction (Williamsport, PA) and went north to Elmira, NY, and then ultimately to Sodus Point on Lake Ontario, deep inside enemy (New York Central) territory. The section between Williamsport and Elmira was abandoned by Penn Central in the 1972 time frame, most probably after Hurricane Agnes in June 1972. (Hurricane Agnes caused the death of many railroad lines by washing out bridges and track -- it was not economical to replace and repair many of these lines and they were subsequently abandoned.)
From Williamsport, the line went to Cogan, Trout Run, Bodines, Marsh Hill, Ralston, Grover, Cedar Lodge, Canton, Granville Summit, Troy, Columbia Cross Roads, then into New York to State Line Junction, Southport and into Elmira. Interestingly, PRR shared its ROW with the Susquehanna & New York Railroad from Williamsport to Marsh Hill. The S&NY had a line intersected the PRR at Marsh Hill, and went between Buttonwood to the west through Marsh Hill and to Monroeton to the east, and connected with the Lehigh Valley at South Towanda. The PRR also shared trackage with the Erie from State Line Junction, NY into Elmira -- the Erie line went to the southwest into Pennsylvania and connected with the New York Central at Lawrenceville, PA. The PRR shared trackage for a short segmet north of Elmira with the LV -- this line is still active from Elmira to Elmira Heights.
Thanks to Matt Gibbons for contributing information about this route.