At least six people have been killed and 60 wounded when a six-carriage Metro subway train rammed into a stationary train in Washington, DC.

Metro officials do not yet know the cause of the collision, but an investigation is underway. Both trains were on the same track headed toward Shady Grove Metrorail station, according to preliminary reports.

A female driver, who was among the 6 killed, was on the trailing train. 

Washington Mayor Adrian Fenty said, "It looks to be the worst Metro accident in DC history. We're going to investigate this and find out what happened."

"This is an incredibly tragic day and our hearts go out to the families of those who suffered fatalities and to those whose loved ones are injured," said Metro General Manager John Catoe, adding, "We are committed to investigate this accident until we determine why this happened and what must be done to ensure it never happens again." 

One train had stopped near a platform and was waiting for permission to proceed when it was hit from behind by the second train, Catoe said, refusing to speculate on whether safety devices intended to prevent such crashes had failed.

According to the Washington Post, the impact of the crash was so powerful that the trailing train was left atop the first train. The daily quoted Mayor Fenty as saying that fire officials were still going through the trains to make sure they had recovered all the bodies.

Image: People work at the scene where two transit trains were involved in a crash in Washington.
Photograph: Yuri Gripas/Reuters