Better to have and not need... UPDATES
a small bottle of hand sanitizer, and small bottle of water. both suggestions are courtesy of my sister, a daily traveler on the DC metro.
i'm going to scratch my words on cell phones; bring the damn things. by way of doc russia comes this suggestion: on your cell phone address list, add an entry ICE-- In Case of Emergency-- with your next-of-kin's phone number. (see also this article in the washington post.) a very good idea, though not without some pitfalls.