by Lucius Kwok
One of the best resources for locating old railroad lines is aerial photographs. I've been using Google Earth for a few weeks and with some practice you can spot lots of old rail lines, even ones that have become overgrown with weeds.
http://earth.google.com/
This is one of the Manayunk bridge in Philadelphia.

http://earth.google.com/
This is one of the Manayunk bridge in Philadelphia.
