OAKLAND, California — A man who police say killed an 18-year-old woman in an unprovoked stabbing at an Oakland train station was arrested aboard a train about a dozen miles away Monday night, officials said. An anonymous tip from a rider on a Bay Area Rapid Transit train led to the arrest of John Cowell, 27, BART Police Chief Carlos Rojas said at a news conference. Cowell — a paroled felon — was arrested at another BART station in the San Francisco Bay Area.