Can you talk to the neighbor that got his back see what reference documents were used ? Have not read all the replies maybe you already did that ? If they don't cooperate you should be able to go to county recorders office or auditors office and look at that info as it should be public knowledge and recorded.

We have a similar piece of property where railroad came through early 1900's and was then abandoned. We had to have it surveyed for a transfer and the surveyor was able to find the appropriate reference materials to establish property boundaries for our land. My friend adjoins up to our property and he is in the same situation as you no clear way to reestablish ownership of previous right of way. He has spent a fair amount of time and money as well and got nowhere. Not sure if there was a difference in how the RR originally obtained the right of way or what caused the different outcome.

In this case if you are occupying or maintaining to the center of the right of way I would think adverse possession would come into play. If you are not and the neighbor is he might have claim to everything up to where you are currently occupying ?

One would think county would be very proactive to make sure ground is having the property tax paid on it by somebody.