After I shot this, he let go of the train and rolled over to talk to me. He wanted a copy of this shot to send to his mom. He says that he rides the trains this way a lot, rolling up and down a few blocks. One of his favorite things to do is to hold on to a train that is turning, then let go as it turns, letting the momentum carry him across the intersection.
I have a suspicion that this is not an approved way to ride the Muni.