Here's one commenter from the washingtonpost.com on the matter:

How is the Washington Post being scooped about a potentially serious metro rail collision? This happened about 45 minutes ago, and it's still in little print at the bottom of the Post web page. Come on guys. Surely you haven't all taken the buy out?

Comes this dispatch, via Dr. Gridlock:

A Red Line Metrorail train derailed at 5:10 p.m. approaching Fort Totten in the Shady Grove-bound side. Trains are turning back at Brookland and Takoma....Metro says that shuttle bus service has been requested to bridge the gap between Takoma and Brookland.

There are reports of injuries. Developing.

UPDATE, 5:35 P.M.: This seems quite a bit worse than a minor derailment. WJLA-TV reports:

Metro confirms two trains have collided on the Red Line between the Takoma and Fort Totten stations. It happened close to the Fort Totten station, a Metro spokesperson said.

D.C. Fire and EMS spokesman Alan Etter said one train was on top of the other train.

This is "developing into a mass casualty event," Etter said. "We're expecting a number of injuries. We're not aware of any fatalities at this point."

Update, 5:46 p.m.: ABC News/News Channel 8 is reporting one fatality. Reporter Brad Bell saw the fatality being taken from the accident scene. At least nine people injured. The fatality appears to be a male. Many passengers still stuck inside metro cars. Fire Department spokesman Alan Etter confirms one dead on WTOP.

Here is the official Metro alert:

"Metro reports that 2 train collided and one train is on top of the other train.  Metro reports massive injuries at this time. The green line and the red line are affected. Further information to follow."

It looks like the accident ocurred just beyond the Fort Totten stop.

5:54 p.m. Update WTOP is reporting that the derailment took place between Fort Totten and Takoma Park:

"The six-car train was heading toward Shady Grove when it derailed and then collided with another train around 5 p.m. Monday near the Maryland-D.C. border."

Metro Official: Passengers still being evacuated from trains.

6:02 p.m. Update: NBC4 reports the collusion occurred above ground, many more injuries:

"Officials have confirmed one fatality. There are numerous other injuries. Passengers trapped in the trains have been using tools to cut themselves from the wreckage.

A witness told Bensen that it appeared that one train car looked like it was on top of another train car.

Trains are not traveling between the Brookland and Fort Totten stations. Shuttle service is being established, but Metro spokeswoman Cathy Asato advised riders to completely avoid the Red Line this afternoon and evening.

Trains are operating between Glenmont and Brookland and between Shady Grove and Brookland stations and from Glenmont to Takoma Metrorail stations for the remainder of the day."

6:06 p.m. Update: Two confirmed dead.

6:11 p.m. Update News Channel 8 reporter re: Witness: One car sheared another car in half. Now with Metro Official: At 5:05 p.m. two trains collided, not sure if dead is Metro employee, rescue workers still getting people off trains. Fire Department has to cut through train to get passengers out.

Front car sheered off.

6:18 p.m. Update: Washington Hospital Center may get most of the victims.

ABC/News Channel 8 reports:

"ABC 7's Brad Bell reports seeing several people with what appear to be broken limbs. He also has seen ambulances lined up."

Witness: Nine serious injuries--internal injuries. Fire Chief Dennis Rubin and D.C. Police Chief Cathy Lanier on the scene. Many people were able to walk off the tracks.

6:28 p.m. Update: TV news channel 8 reporting rescues may have been completed. Rescue workers now working on passengers. Civilians working on passengers, performing first aid. Good point: Mass Transit in need of funds.

6:30 p.m. Update: Two students appeared to have broken legs.

6:40 p.m. Update: Another picture, this one from channel 9 via Twitter:

6:41 Update: MOCO ambulances and emergency vehicles are coming on the scene.

6:43 p.m. Update: WCP  is reporting from the scene.

6:45 p.m. Update: From WJLA: People are still on the train. A bad sign: Medical Examiner is coming on the scene. Doors of train jammed requiring emergency personnel.

6:46 p.m. Update: Via twitter: One resident reports getting a warning regarding Metro Trains just before crash:

"Got a DC alert at 4:42 about mechanical issues and trains sharing a track. Crash didn't happen yet. Foreshadow?"

Correction: It was at 4:39 p.m. between Tenleytown and Van Ness. It was for mechanical problems.

6:56 p.m. Update: U.S. Park Police helicopter used to evacuate victim. One of the two dead was a driver on one of the trains from NBC-4.

Mayor Fenty to hold press conference at 7 p.m.

7:03 p.m. Update: WMATA has issued a press release:

"Two six-car Red Line trains collided at 5 p.m. today, Monday, June 22, resulting in two fatalities, including a female train operator. There were also several injuries, many serious, according preliminary reports.

Metro officials advise that people avoid the Red Line the remainder of the day. Trains are operating between Glenmont and Silver Spring Metrorail stations and between Shady Grove and Rhode Island Avenue Metrorail stations for the remainder of the day. (Please note that this is a change in locations.)

Shuttle buses are operating between Silver Spring and Rhode Island Avenue Metrorail stations to help customers get around the incident.

Emergency officials remain at the scene.

'We are extremely saddened that there are fatalities as a result of this accident, which has touched our Metro family. We hope to have more details about the casualties later today. Our safety officials are investigating, and will continue to investigate until we determine why this happened and what must be done to ensure it never happens again,' Metro General Manager John Catoe said."

7:06 Update: Fenty press conference moved to 7:15 p.m.

7:08 Update: More than a 100 injured re: NBC-4. A bus with 24 "walking wounded" en route to George Washington University Hospital.

7:09 Update: Fenty press conference begins: "We are cooperating fully with WMATA," Fenty states. Fire Chief Dennis Rubin about to speak. "A little bit after 5 o'clock we responded," Rubin says. Called in fire departments from surrounding counties. Treated 70 patients including two with life-threatening injuries.

Four dead. Including one female train operator. This is the deadliest accident in Metro history. There was a previous crash in 2005 which caused scores of injuries. Two metro workers were killed in a 2006 crash. Wiki has a good rundown of previous accidents.

You can follow the liveblog of the press conference.

Officials still searching train for casualties.

7:19 Update: Fenty tries to end press conference. Official: If you are trying to find a loved one please call: 202-727-9099. Do not go to the scene.

7:24 p.m. Update: Chief Rubin says this was a three-alarm emergency that involved 200 rescue workers.

7:30 p.m. Update: Our own Mike DeBonis is reporting that the train that got hit had been stopped for 10 minutes. We will have a full report from our man on the scene soon.

7:32 p.m. Update: David Corn is taking some pictures of the crash:

Here is another from Corn:

7:37 p.m. Update: WMATA updates its press release:

"Two six-car Red Line trains collided at 5 p.m. today, Monday, June 22, resulting in four fatalities, including a female train operator. There were also several injuries, many serious, according preliminary reports.

Metro officials advise that people avoid the Red Line the remainder of the day. Trains are operating between Glenmont and Silver Spring Metrorail stations and between Shady Grove and Rhode Island Avenue Metrorail stations for the remainder of the day. (Please note that this is a change in locations.)

Shuttle buses are operating between Silver Spring and Rhode Island Avenue Metrorail stations to help customers get around the incident.

Emergency officials remain at the scene. Metro officials are also working with the National Transportation Safety Board."

7:40 p.m. Update: From DeBonis: Reporters have been moved off the scene. They've been moved 100 yards from crash site.

7:45 p.m. Update: Scores of rescue workers lined up along 2nd Street: NBC-4. Read WTOP's story here. WJLA updates its story as well. WMATA updates its press release.

7:58 p.m. Update: Two traumas are at G.W. The bulk of the victims are there via NBC-4.

Unofficial count of dead now 7--WUSA. Then there's this unconfirmed account via Twitter.

8:07 p.m. Update:From the Washington Post's report updated roughly 15 minutes ago:

"No details were available about the cause of the crash, the third time in the last 15 years involving the collision of two Metro trains. The last was in November 2004, when a Red Line train rolled backwards down a steep stretch of track, and smashed into another train at the Woodley Park station. Twenty people were injured in that crash.

'There will obviously be a very thorough investigation into what caused this derailment,' Metro spokeswoman Farbstein said of today's crash. She said there were 'many serious injuries' and that passengers were being taken to hospitals. 'Our hearts are with the families of those who have passed,' she said"

8:12 p.m. Update: WJLA reporting six dead:

"A Metro train slammed into the back of a stopped train on the Red Line during the height of rush hour Monday afternoon, killing six people and injuring more than 60 others in the deadliest crash in the transit agency's 33-year history. ABC 7 News confirmed there were at least six fatalities in the crash, including a train operator. Officials have only cofirmed four."

8:17 p.m. Update Fire Department via Twitter reports transporting 42 injured.

8:27 p.m. Update: Fenty is back before the cameras. He confirms six dead. But he cautions that is a "working number."

Rubin hails heroic efforts of D.C. fire fighters. Two fire fighters sustained minor injuries. Lanier says a "reunification center" has been set up. Call 311 if you are looking for a loved one. We are reporting out the press conference in full.

8:36 p.m. Update: WaPo finally updates its story to include the six dead.

8:56 p.m. Update: New number to call if you are looking for loved ones: 202-737-4404.

9:05 p.m. Update: Councilmember Jim Graham: Car "leapfrogged" other car. Lack of info regarding crash is due to the state of the cars. 76 injured.

9:07 p.m. Update: DeBonis reads through old NTSB reports regarding previous Metro crashes---issues of the car construction could be at play here.

9:12 p.m. Update: Check out our pictures from the crash site.

10:32 p.m. Update: This from unsuckdcmetro: "Just got a tweet from WMATA. "disruption cleared" FAIL #wmata"

11:30 p.m. Update: Darrow Montgomery posts some photos that capture the aftermath of the crash; Lanier, etc. look stressed out.

11:35 p.m Update: NBC-4 reporting that rescue workers are still looking for people in the twisted wreckage.

11: 45 p.m. Nine confirmed dead.

---By Mike DeBonis and Jason Cherkis

*photo courtesy of Channel 9 News, WNBC-4