One commuter train rear-ended another stationary train on Washington, D.C.'s subway system, killing at least four people and injuring dozens. (CBC)At least six people were killed after two trains collided Monday during rush hour in Washington, D.C.'s transit system.

"It is my present understanding that this would then be the deadliest accident in the history of our Metro train transit system," said the district's mayor, Adrian Fenty, during a news conference at the scene.

The accident occurred at about 5 p.m. ET when a six-car commuter train rear-ended a stationary train on the Red line between the Takoma and Fort Totten stations. The route, Metro's busiest, shuttles thousands of commuters each day between the city and the D.C. suburbs.

One of the two trains was stopped on the tracks near the Fort Totten station, waiting for a train to clear the station ahead, when the other train plowed into it from behind. The operator of the second train died.

Officials had no explanation for the accident, which occurred on an above-ground portion of the tracks. The National Transportation Safety Board is leading the investigating and sent a team to the site of the crash.

The incident was "developing into a mass casualty event," said Alan Etter, spokesperson for the Washington Fire and Emergency Medical Services Department.

Firefighters were on the scene where part of one train could be seen lying on top of another.

District of Columbia Fire and Emergency workers were at the site of a rush-hour collision between two Metro transit trains in northeast Washington, D.C., on Monday. (Pablo Martinez Monsivais/AP Photo) Emergency crews were using heavy equipment to cut apart the trains to get people out, Etter said.

Several people were seen being taken on gurneys from the wreckage to nearby hospitals. Some people remained trapped in the trains after the doors jammed on impact, and rescuers were working to free them.

"We don't know the number of injuries yet; we know they've taken over 60 people off the train," Metro general manager John Cato said.

Fire Chief Dennis Rubin said rescue workers treated 76 people at the scene and sent some of them to local hospitals, six with critical injuries. A search for other victims continued into the night.

Passenger Jodie Wickett, a nurse, told CNN she was seated inside one of the trains using her cellphone when she felt the impact. She said she sent a message to someone that it felt like the train had hit a bump.

"From that point on, it happened so fast. I flew out of the seat and hit my head." Wickett said she stayed at the scene and tried to help. She said "people are just in very bad shape."

"The people that were hurt, the ones that could speak, were calling back as we called out to them," she said. "Lots of people were upset and crying, but there were no screams."

The second deadliest incident on Washington's Metro system was a derailment on the Orange line between the Federal Triangle and Smithsonian stations that killed three people in January 1982.

The two trains were travelling above ground on the Red line, the Metro system's busiest route, when they collided Monday during rush hour. (Washington Metropolitan Area Transit Authority)The last major commuter crash in the United States was in September 2008 in Chatsworth, north of Los Angeles, when a commuter train collided with a freight train. Twenty-five people died and 134 were injured in that crash, the worst train accident in the U.S. in some 15 years. Investigators of that accident found the engineer of the commuter train was sending text messages on his mobile phone prior to the crash.

With files from The Associated Press