VALHALLA, N.Y. — An investigation into what caused a fiery crash that killed a motorist and five rail riders is focusing on how a mother of three described by friends as safety conscious ended up between two crossing gates in her SUV as a commuter train barreled toward her.

“The big question everyone wants to know is: Why was this vehicle in the crossing?” Robert Sumwalt, a National Transportation Safety Board vice chairman, told reporters in suburban New York a day after the deadliest accident in the 32-year history of Metro-North Railroad, one of the nation’s busiest commuter railroads.

NTSB investigators were working today to examine the tracks, interview the crew and find out whether Ellen Brody’s Mercedes SUV had a data recorder of its own.

Service on the Harlem Line resumed today with delays, as trains slowed down through the crash site area.

Brody, 49, was driving home from her jewelry-store job Tuesday night when a witness said she got out of her vehicle to check after a guardrail came down on top of it. She then got back into the car, driving forward onto the tracks just before she was struck by the train, motorist Rick Hope told The Journal News.

“She wasn’t in a hurry at all, but she had to have known that a train was coming,” Hope said, adding he backed up and motioned for her to do the same.

The crash happened in the dark in an area where the tracks are straight but drivers exiting or entering the adjacent Taconic Parkway had to turn and cross them.