Washington, D.C., Police Chief Cathy Lanier advised family members of passengers to call (202) 727-9099 for more information. A secondary number issued by local district officials is (202) 737-4404.
Past accidents on Washington's Metro have included a January 2007 train derailment near the Mt. Vernon Square Station on the green line, in which 23 passengers were transported to local hospitals for treatment and released; a November 2006 accident where a train struck and killed workers in Alexandria, Va.; and a May 2006 accident where a train struck and killed a worker near the Dupont Circle Station.
In November 2004, two trains collided at the Woodley Park-Zoo/Adams Morgan Station, and about 20 people were taken to nearby hospitals.
Elsewhere, a train derailed in Rockford, Ill. earlier this month, prompting the train cars holding flammable ethanol to explode.
Last year, 25 people died in Southern California when a Metrolink commuter train and a freight train collided. The Metrolink train engineer, who died in the head-on collision, was sending text messages just before the accident.
ABC News' Devin Dwyer, Jason Ryan, Jack Date, Bret Hovell, Matt Hosford and The Associated Press contributed to this report.