The National Transportation Safety Board said the smoke in the Metro tunnel near L’Enfant Plaza was caused by electrical arcing, which occurs when electricity escapes its prescribed path. Metro has had problems with arcing in the past, particularly in areas that have water leaks. Read related story.
Louis Sanders, director of technical services for the American Public Transportation Association; National Transportation Safety Board, WMATA | Bonnie Berkowitz And Alberto Cuadra / The Washington Post 