Tagger by Roger Lazoff
This older guy, short and bowlegged, is walking down the traincar, using two canes as he goes, watched over by a much taller woman. It's Norman Mailer and his wife Norris Church, on a leg of their trip to their house in Provincetown. Mailer is still feisty, and seeing the kid with the cans, says, "I wrote the book on tagging, in '71 or so, 'The Soul of Graffiti,' my homage to the ordered chaos then underground." The kid doesn't know who is talking about what, but he's interested.
Mailer goes off on a rant, points about how to participate in revolution, and then, plunk, he seems to have a heart attack. Tagger revives him by taking apart one of his cans and using the propellent in some emergency medical way. Norman comes back and continues on his rap as the train stops and he limps out with Norris. Tagger gets a picture of the writer floating in space, words pouring out of him.