They were used when section crews would replace all the ties and rail. That was way back when, stopping in the 70's could be right, but I'm thinking earlier than that. You might still find them on branch lines, or on old track no longer used out there somewhere. I at one time would pull some, and I gave them all away, along with battery oil bottles that MIGHT still be found around where signal's used to be. At one time there might be a "section crew" maintaining about 10 to 15 miles of track, with housing and family's living with them, of then there would be another section crew with housing and family's etc. It was all jointed rail then, if I remember about 33' in length, whereas now on "main line" they come in 1/4 mile pieces, (or "sticks" ) Date nails are cool, and there are different types, depending on what rail road it was. As far as fence post, ranchers and farmers loved them, used that way, they could pretty much last forever. Nice collection "Old Pueblo". Spent time working on Navajo steel gangs in the mid 80's in your area, loved wandering around in the desert looking for stuff.