My understanding is that they do not have the right to ask for paperwork, that your word is all that is necessary.
I have both handicapped parking plate and tag, which required paperwork. My doctor insisted I get them several years ago, as well as a wheelchair to save walking through parking lots at stadiums, museums, airports, anything that requires a long walk and standing. I did ask him to write a letter stating that I was permantly disabled for the national park access pass, but they have never asked to see it. I was told they aren't allowed to ask for proof. I actually begged the ranger at Zion to look at it just so I hadn't wasted my doctor's time. I do not think the airport personnel are allowed to ask for proof either, so I don't see how a train conductor would.
It does take a thick skin to demand these rights, particularly if you aren't in a wheelchair. Fortunately my husband has one, so he does the talking and I just sit and look slightly addled which is very easy for me.
I hope it wasn't a long ride. I know you, like me , are very tall. I have given up going to certain venues because of my knees being bent at an excruciating angle in the effort to cram as many seats as possible, the Comcast Center in Philly being my chief nemesis. We may explore their handicapped seating next time TSO comes to town.