I forgot all about this! When I was a teenager, I lived near Hershey PA. I used to walk the old railroad tracks and would find large greenish colored marbles. A buddy's dad told us these were actually made when engines would need traction, there where "jets" of sand sprayed on the wheels to help with slowing the train, etc. The results were small glass balls that would easily fall to the gravels between the wooden ties. I gave these to my younger brother back in the 1970"s. My brother recently gave them back to my teenaged son. Sorry I have no pics yet. Thought you marble collectors may want to try walking the tracks in your area, please be careful if you go! Good luck. 
