My first question to them would be, please give me information about this law.
I've done a _LOT_ of searching and have never seen how a RR would be any different then any other land. It's as you mentioned... if you are not on their land then it comes down to not flying in a reckless manner. Call me crazy but I don't see how a drone is going to pose much of a danger to a train. I just can't see anyone claiming it could.
My recommendation if you want to play it on the safe side is to contact the office and/or those people that came out to your home and ask them to cite the law you were breaking. Once they fail to do this document:
https://www.faa.gov/uas/regulations_policies/media/UAS_Fact_Sheet_Final.pdf
I have this document printed up and carry it with my Phantom at all times. It lets all local people know that the FAA and only the FAA can regulate airspace. Those locals are more then welcome to report drone flight to the FAA. Keep in mind (and it appears from your post that you know this) that local people can regulate their own _land_. So if you are standing on their property they can regulate you... but not the drone in the air.