According to a story on the SF Appeal:
The prank artists plan on acting normally, and not interacting with any of the other pant-less riders in an attempt to confuse other passengers on the train who are not in on the joke.
Along with New York City and San Francisco, similar events are planned in Atlanta, Boston, Chicago, Cleveland, Dallas, Denver, Honolulu, Los Angeles, Philadelphia, Phoenix, Pittsburgh, Salt Lake City, San Diego, Seattle, and Washington, D.C.
Hang on, Salt Lake City has a subway?
Anyway, I currently have someone in my household who HATES wearing pants. Sometimes I have to chase him around the room for 10 minutes before he'll put them on. In short, Elliot would love this event.
For that reason, I'm going to do everything in my power to keep him away from it.
Elliot can never know that non-pants wearing is a valid lifestyle choice. It's just too dangerous.