Hundreds of feet of electrified rail skewered the first two carriages of a New York commuter train in a collision with a car at a railroad crossing, a federal investigator said on Wednesday, describing the area's worst rail crash in decades.

Investigators were focused on why the car was stopped at the crossing near the suburb of White Plains north of New York City before the Metro-North train crashed into it during Tuesday evening's rush hour, pushing the vehicle about 1,000 feet down the line.

The rail broke into long pieces, penetrating the first train carriage as a fire broke out, apparently fueled by gasoline in the vehicle's fuel tank, gutted the rail car's interior, he said. At least one section of the electrified, or "third," rail also entered the second carriage near its ceiling.

"This third rail is just basically piling up inside that first train car," Robert Sumwalt, a member of the National Transportation Safety Board (NTSB), said at a news conference ahead of a week of gathering evidence and interviewing witnesses.

Sumwalt said the NTSB expected to release data from the recorder on the train on Thursday.

Five train passengers and the woman who was driving the Mercedes sport utility vehicle that was stuck on the tracks were killed. Investigators said they do not yet have an explanation for how the vehicle, which officials had earlier mistakenly identified as a Jeep, became stuck on the tracks.

Metro-North, run by the state-controlled Metropolitan Transportation Authority, had four high-profile accidents in 2013 that led to a safety assessment by the Federal Railroad Administration (FRA).

In a March 2014 report to the U.S. Congress, the FRA criticized the nation's second-largest railroad for a "poor safety culture" and "ineffective training".

The NTSB released a report late last year that also identified common safety issues, but Sumwalt said Tuesday's crash may be unrelated.

"I would be very cautious with trying to draw a nexus with what may have happened with Metro-North in the past and this accident," he said.

The crash appeared to be the deadliest rail accident in the New York area since March 1982, when nine teenagers in a van were killed when a train crashed into them at a crossing in Mineola.

A Metro-North train derailed near the northern edge of New York City in December 2013, killing four people and injuring 70. In May 2013, two Metro-North passenger trains collided in Connecticut, injuring more than 70.

Some 650 passengers regularly take the 5:44 train, which carries commuters through some of country's wealthiest suburbs.

Fifteen people were injured, New York State Governor Andrew Cuomo said. One passenger remained in critical condition and another passenger in "serious" condition on Wednesday afternoon at the local trauma hospital along with six other patients with less serious injuries, Westchester Medical Center officials said.

The Mercedes' driver was identified as Ellen Brody, a 49-year-old jewelry-shop worker with three children, according to Paul Feiner, the Greenburgh town supervisor.

"She was not a risk-taker in terms of safety," Feiner, who described himself as a family friend, said in a telephone interview.

One of the killed passengers was identified as Eric Vandercar, who worked at Mesirow Financial, according to a statement by his previous employer, Morgan Stanley. Another was Walter Liedtke, a curator of European paintings for New York City's Metropolitan Museum of Art, the museum said.

Collisions at grade crossings in the United States have declined by more than 40 percent to 2,091 in 2013, from 3,502 at the turn of the century, according to data compiled by the Association of American Railroads and the FRA.