There is a large collection of Erie Railroad maps and drawings online. These show rail lines in the Mahoning Valley and some show locations of industries that have long since disappeared from the face of the earth. I finally found the exact location of the Girard blast furnace ( the one Joseph G. Butler, Jr. was manager of) and its under I-80. There is one that shows the locations of all of the Erie's customers in the Valley as well.
http://www.rr-fallenflags.org/el/dwg/draw.html (http://www.rr-fallenflags.org/el/dwg/draw.html)
Well worth a look for those of us interested in the industrial history of the Mahoning Valley.