Those are usually not surveyed in with any detail, they are installed by maintenance of way crews. As Norm mentioned, more often than not the actual TS, SC, CS, ST points will fall off the ties. They are a good point to start when regressing the track centerlines.
Anything you set in the ballast is going to be destroyed by tamping and or tie replacement. If you want it to stay put it needs to be in the sub-ballast.
They are evidence, weight them as you will.
Me. "What's the difference?"
T.C. Carroll "It's the difference between right and wrong!"
Not knowing where you are may matter. Some additional thoughts:
In my part of southeast NY, one cannot work within the RR right of way without having the crew pass the RR safety course, and then applying for and having the RR flaggers and safety people there while you are working. I have never done it, but I believe it is about $8,000 a day for the flaggers. Fines for working in the ROW without the permit and flaggers are substantial.
In a Continuing Education course on this I heard some right of ways were monumented at the centerline PC and PT with substantial stone or concrete monuments. Over time, the RR started using a machine that rolls down the track lifting and resetting the ballast and ties, a process that cuts and moves the centerline monuments.
As said, tracks move, and the middle of 2 sets of rails may not be the centerline.
Some ROW points are monumented with a vertical piece of rail, which really makes the finder sing.
I have had good luck using building offsets from old surveys that were performed when the monuments could be located and the crews were permitted to work in the ROW.
Good luck!
Ken