Whoever is doing this probably has plenty of time on the train during which they're not being observed. Maybe one of the folks who cleans the train?
Might be a bit tough to see, but someone cut the heads off pictures from one ad and pasted them onto this ad for The O.C.. The heads are those of Dr. Phil and Oprah. Couldn't tell who the other folks were. (One looks like Olympia Dukakis.)
You simply can't do this on the train in full view of the passengers. And unless you take a train that runs at 3 AM or something, one would never run into a situation where one could pull this off without having somebody see something, which makes me think it's probably one of the cleaning folks.