Investigators in the Bronx train derailment are poring over engineer William Rockefeller's every move before the crash -- from his breakfast choice that morning to his actions seconds before the cars left the track.

Among the many mysteries investigators hope to solve is a crucial question: Why was the train barreling down the tracks at 82 mph -- almost triple the 30-mph speed limit in that area -- before it left the curved stretch of track?

Investigators interviewed Rockefeller Monday and will continue speaking with him over the next few days as they pursue answers.

A law enforcement source said Rockefeller's statements would be compared with information from the train's data recorders.

"The data tells the story of how this happened," said the source. "It corroborates or debunks."

Investigators are looking at the 20-year MTA veteran's training and his safety record. They want to know how much sleep he got and the last time he had used his cellphone, which they are examining.

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Rockefeller, 46, was tested for drugs and alcohol after the accident, but results are not yet available, said National Transportation Safety Board member Earl Weener.

To his friends and colleagues, Rockefeller is known as someone who loves his work driving trains. He has a passion for railroads, their history and the safest way to operate them, said Donald Stone, a friend and neighbor of Rockefeller in upstate Germantown.

"Bill is a true professional who knows those trains better than anyone," Stone said. "He's as good as they come -- a railroad history buff and a guy who's big on safety."

Anthony Bottalico, executive director of the Association of Commuter Rail Employees, Rockefeller's union, said the 11-year veteran engineer "is totally traumatized by everything that has happened," but is cooperating fully with investigators.

"He's a sincere human being with an impeccable record, that I know of," Bottalico said. "He's diligent and competent."

Neighbors in Rockefeller's Germantown neighborhood described him as a car and motorcycle buff who loved tinkering with engines and other machinery.

"He loved looking at how things work and what makes them go," said neighbor Brian Costello, 47.

"He's got an engineer's brain. . . . We're just hoping he comes out of this OK, because there's been enough tragedy already," he added.