Not sure of when something becomes a historic artifact. I essentially leave everything I'm finding right where it is. I'll occasionally move items to get a better photo angle.
Besides being illegal, if everyone takes all the artifacts then there won't be anything to see for the next guy. Plus, once you have it home it only has a connection to you and once you are gone it most likely will get thrown in the recycling bin.
I used to collect artifacts years ago, but began to donate them to various museum and find now its better to just take pictures. I hope by documenting these sites it allows all of the armchair railfans to see what's out there. If it became apparent people are using my threads to raid these archaeological sites of artifacts, I'd stop posting this information...