None of the above. I will smack it out of the air smashing it into the floor because some idiot was flying his RC helicopter inside the train.
We have flies that work their way into the cockpit once in a while. Do they add to the gross weight of the aircraft? Only when they are flying or when they are sitting? Lindbergh wondered the same thing on his transatlantic flight.
It isn't connected to the train in any way; what the train does has no effect on it. It makes about as much sense as saying "a mini helicopter is hovering 4' in the air, you take a hula-hoop, turn it vertical, hold it, and run past the helicopter, making the helicopter go through the hoop...what happens to the helicopter?" The answer is: nothing. Same concept, juts larger scale because we are talking about a train rather than a hoop.
There is a young, science dude who has a lot of vids on YouTube. He has done that one (in a minivan). Blew my mind.
It's just like the cargo van that I use for work during the winter, I'll heat it up in the mornings, but a soon as I get to the stop sign at the end of the hood, I hit the brakes, and all of the cold air comes blasting forward in to the cab.