In Metro-North Crash, 400 Feet of Third Rail Sliced Through Train At least six people were killed in the crash.

 -- Some 400 feet of the electrified third rail sliced through the first car of a Metro-North train after it plowed into an SUV stalled on the tracks, burning the entire interior of the car and killing five passengers, the head of the National Transportation Safety Board investigation confirmed this evening.

The accident, which happened during the Tuesday night commuter rush near Valhalla in suburban New York, was caused by the train hitting a Mercedes SUV that, "for whatever reason," was on the tracks, NTSB vice chairman Robert Sumwalt said.

The car was "pushed about 1,000 feet down the tracks" and "during that time, the third rail penetrated" the first passenger cabin of the train, Sumwalt said. The third rail, which supplies power to the train, began breaking apart in 80-foot segments which started to pile up in the first cabin, but a section also went into the second car as well.

Sumwalt said that the crash caused a fire and explosion, and "initial indications are that the fire was fueled by gasoline from the SUV." It was not immediately clear how long the power stayed on when the rail entered the train car, but the NTSB said the rail is designed to deactivate if it is separated.

Sumwalt said that the NTSB investigators, who arrived this morning at the crash site about 20 miles north of Manhattan, will likely stay in the area for the next five to seven days collecting witness testimony and what he classified as "perishable" evidence, meaning information that may disappear or disintegrate over time.

They have already downloaded the recorders that are in place on the tracks and are calling on witnesses to come forward and share any information they have about the fatal crash.

Earlier today, New York Gov. Andrew Cuomo spoke about the tragedy. “There are six people who left home and said ‘goodbye honey’ and never came back,” he said. He called the accident a “truly ugly and brutal sight.”

Co-workers later identified the driver of the car as Ellen Brody, a mother of three from Edgemont, New York. Among the dead on the train were Joseph Nadol, 42, of Ossining, Robert Dirks, 36, of Chappaqua, Walter Liedtke of Bedford Hills, Eric Vandercar, 53, of Bedford Hills, and Aditya Tomar, 41, of Danbury, Connecticut.

Cuomo added that the electrified third rail of the track buckled and pierced the front train car.

“You had a metal rail going right down the train like a ramrod,” Cuomo said. “That’s what caused massive destruction.”

Twelve people injured in the crash were taken to Westchester Medical Center -- and as of early this afternoon, one was in critical condition, one was in serious condition, four were fair, two were good and four had been discharged, hospital officials said. Injuries included lacerations, contusions, crush injuries, fractures, smoke inhalation and burns.

Westchester County Executive Rob Astorino said today that all but one of those killed were charred beyond recognition. Families were gathering around 10 a.m. at the medical examiner’s office to make positive identifications, but Astorino believed medical and dental records would be needed.

“That train had so many flames in it, it was so engulfed, the inside of that first car is just melted and charred," Astorino said.

Citing an eyewitness quoted by local media, Astorino said the preliminary, unconfirmed indication was that the railroad crossing gate came down on the vehicle involved in the accident, the female driver got out to put the gate up, and then she drove forward instead of in reverse and into the path of the oncoming train. He noted that traffic may have been backed up in the area because of another accident on the Taconic State Parkway.

Cuomo told ABC News in a radio interview that “it would be apparently inexplicable” if the driver pulled forward onto the track and into the path of the oncoming train.

“If there’s a lesson to learn we’ll learn it,” Cuomo said. “But sometimes accidents happen and human beings can act randomly.”

Initially, officials reported the car involved in the accident was a Jeep SUV, but they later said it was a Mercedes.

The National Transportation Safety Board launched a go-team to investigate the accident, which is the deadliest tragedy in the history of the railroad.

NTSB board member Robert Sumwalt said this morning that officials have secured event recorders for the train itself, as well as for train and highway signals near the crash, and the agency has sent experts to analyze the recorders. The NTSB team also includes technical experts who can examine areas such as track and highway factors, emergency response and fire science. The NTSB expects to gather evidence at the scene for five to seven days, starting with the most perishable evidence, for an investigation that could last a year.

“Our goal is not only to find what happened, but to find out why it happened,” Sumwalt said. "We go there and we cast a wide net. ... At this point, everything is on the table. Nothing is off the table.”

Angelo Ortiz, one of the first paramedics on the scene, had trouble putting the accident into context hours afterwards.

"What I first saw was the glow of the fire down the road. And as I approached I was in disbelief when I saw the fire was coming from the car that was completely engulfed. When I knew it was the train that was on fire as well, that's when I realized that this is probably the worst tragedy I've ever responded to," Ortiz said.

"It was incredibly, just very tragic. That's all I can say. I'm still in disbelief,"

Rick Hope, a driver who said he saw the train hit the SUV, said he saw the events preceding the accident that Astorino and other officials described.

“She looked very calm and she was taking what I thought was an awful long time because I'm thinking, ‘The clock is ticking here.’ The lights are flashing. The gate's down. You don't have much time,” Hope told ABC News.

The Associated Press contributed to this report.

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