Rail is rated for a certain number of billions of tons. The only place you're going to find old rail on a very active railroad is on a not very active branch or siding. Otherwise, it will have hit its billions of tons, and been replaced.
Well, after several years of saying I'm going back to investigate, I finally did. But I forgot to bring my chalk witch Russ suggested. So I have to go back again.
The missing rail that I reported awhile ago, has been replaced with a "new one." Rails from the main line, were in bad shape and dated 1902. Then, newer rails were installed leading up to the transfer station. Those were dated 1922. I could not locate the 1881 rail. There were two rails on the same side that were just too corroded to read. If I had the chalk, I might have been able to read it. Also, the oldest rails closest to the switches, were buried in ballast and would take a serious effort to clear. I don't think that would look to good to a passing train crew. Which may only be the B&P guys at 930pm.
With having a tenant with kids, I have plenty of chalk laying around the yard now so that will be my next project.....again.