This is a strange occurrence here in the City. Luckily, nobody got hurt. Somebody from the railway should investigate how this could happen. If you visit SF and ride the local trains here, make sure someone is driving it before you board.
http://abclocal.go.com/kgo/story?section=news/local/san_francisco&id=9324618 http://abclocal.go.com/kgo/story?section=news/local/san_francisco&id=9324618