There's an old abandoned rail road grade behind where I live that was a spur to a coal mine and all the track was pulled, but many of the track bolt left behind look almost exactly with the same amount of deterioration as yours. The track spikes on the other hand are not as rusted away as the track bolts and this makes me think the old bolts were made of higher carbon steel?. What I know about the old RR grade is it was most likely put in around the 1880's. and ripped up during WW2 for scrap, as the coal mine did operate in to the later 1930's and shut down to a fire I believe. The 2nd half of the 19th century is my guess on when your bolt was made.