If you consider what went on in the 20th Century, I think a reasonable conclusion is that 60# or smaller rail was long gone by WW2 from most track that could handle early 20th century locos, especially here in New England as all those bucolic branches saw their manufacturing customers move away. As mainlines were upgraded to handle the heavier equipment the 80# stuff was used to relay most secondary track in the 20's, or as cheap replacement in the depression, and then any remaining small stuff was taken up during the scrap drives in the war.
Therefore anything still on the ground today and not in use is at least 80# and likely belongs to a state. The only 60# stuff I've come across is about 200' of guard rail off a Guilford/Pan Am bridge, and about 60' left on a siding in Concord, MA, buried under the leaves.