What happens to the crossing lights when they are replaced with newer lights?
Can they be bought by the public/town/historical sites?
How can they be tracked down (no pun intended)

maybe by the town or historical sites. l do not know about the public. you can always try though

The railroads have no use for them and normally discard them.
If you contact the signal supervisor at a railroad near you, they should be able to help.
Crossing lights are not replaced very often though. Seems like it was about 4 or 5 years ago that most crossing lights were required to be replaced with larger ones so there might be some of the older ones left in a scrap pile somewhere.

There r individuals & companies that buy them up & offer them for sale.

If u do an internet search on ''railroad signal for sale,'' u will find all u could possibly want. There r also usually a number of them for sale on eBay.

American railroads & state transportation departments r currently undergoing extensive programs of improving highway crossings and, in many cases, eliminating grade crossings. One result is a large number of old crossing signals & related equipment for sale.

As one other answerer mentioned, it is also sometimes possible to buy old equipment directly from the railroads at demolition sites or workshops.

Our club purchased several of the old ''Wig Wag'' Style crossing light systems. We r currently using them for our grade crossings on our Club is Railroad.

FRA has approved them for us because we only operate our trains once a week on Sunday is & we have been classified as a Class 3 Railroad Historical Landmark.

However we were only able to use these on the driveways of peoples houses that our tracks cross just before entering the highway. On the major crossing we had to use the newer gate arm system that is currently in use.

We also use the semifore signals along our line too. Those r the ones where they have the little flag arm that raises & lowers with the associated lights.

The Wig Wags were obtained from the City of Santa Cruz, where they were forced to upgrade their crossing guards, so we contacted them, & were able to purchase them.

If u contact ur local railroad, they may have some of that equipment still left in a local scrap yard they use for parts. But l would not be too optimistic about that.

Also check E-Bay for some of those items.

l hope that helps u out. Good Luck!