Two trains have collided in Washington DC's Metro. One train is on top of the other, officials say, in between Takoma and Fort Totten on the Red Line. Six people are confirmed dead. Unconfirmed: up to seven dead.

Update 3: DC's ABC-7 reports six dead at the scene, still possibly seven deaths, and two still in critical condition at the hospital.

Update 2: One train was stopped on the tracks, waiting for clearance into the Fort Totten station. The second one "plowed right into the tail end of the first train without slowing down," according to a reporter at WJLA. The driver of the second train is among the confirmed dead. 70 have been transported to hospitals, with two in critical condition. Four officially dead, up to seven may be dead altogether (meaning, probably, that there are still three people trapped in the wreckage).

(Mother Jones Washington bureau chief David Corn is taking photos of the wreckage with his iPhone.)

Update: Three people possibly still trapped in a train car. More news photos here, eyewitness reports here. A train operator is one of the dead, and while the logistics of the crash are still unclear to us, the eyewitnesses seem to be saying that a speeding train ran into a stopped one. In addition: the Metro may have been single-tracking on the Red Line this afternoon due to "mechanical problems." If you're wondering if a family member was on one of the trains, you can reach the DC police at 202-727-9099.

The Red Line is the busiest line on the Metro, and this crash—reportedly initially caused by a six-car train derailing and colliding with an oncoming train—happened around 5 pm, at the height of rush hour.

D.C. Fire and EMS spokesman Alan Etter tells WTOP rescue crews are setting up for the "possibility of mass casualties."

There's full, live coverage available via streaming video here. (It pops up.) DC police and fire are still rescuing people from trains. The Red Line is elevated at the site of the crash, and ambulances are unable to reach the tracks.

Top photo: MyFoxDC. Additional photos: Inside transit.