He was asleep at the switch.

The engineer of the Metro-North train in Sunday’s disaster dozed off moments before the deadly derailment, his own union chief admitted.

“He basically nodded . . . He had the equivalent of what we all have when we drive a car — that is, you sometimes have a momentary nod,’’ Anthony Bottalico said of engineer William Rockefeller.

“How long that lasts, I can’t answer that. Only Billy can,’’ said Bottalico, head of the Association of Commuter Rail Employees.

“He caught himself, but he caught himself too late . . . He put the train in emergency, but that was six seconds prior to derailment.”

Later, Rockefeller’s lawyer said his client, while talking to investigators, described what happened to him as a “daze.”

Attorney Jeffrey Chartier accompanied the engineer to the interview and said the “daze” was almost like road fatigue.

Authorities said the train was equipped with a “dead man’s pedal,” which activates the emergency brakes if the operator’s foot is removed.

The pedal is to “assure there is an operator paying attention,” said National Transportation Safety Board board member Earl Weener.

“The dead man’s switch is in the cab car for the left foot, and basically, the operator is sitting or standing, holding his foot down on that,” Weener explained.

“If he releases that, then the train goes into service-braking.’’

A whistle went off warning Rockefeller that the train was going dangerously fast just before the derailment, sources have said.

The emergency brakes were automatically activated — likely because the seven-car train, pushed by a locomotive, was already starting to derail just north of the Spuyten Duyvil station in The Bronx, a source said.

That’s when the disoriented Rockefeller made his first attempt to control the runaway train. He pushed the throttle into neutral and then tried applying the brakes, the source said.

Rockefeller claimed at the crash site that the brakes malfunctioned, sources said.

But “simply put . . . there is no indication that the brake systems were not functioning properly,” Weener said.

Rockefeller, who departed on time from Poughkeepsie at 5:54 a.m., was able to stop the train at nine stations before the derailment.

Officials said he finished his shift the day prior with plenty of time for a good night’s sleep.

Bottalico said Rockefeller, 46, “never said anything about not getting enough sleep” — but he noted that the engineer had just switched on Nov. 17 from the night shift to working in the wee hours.

“So he did have a change in his hours and his circadian rhythms with regard to sleep,” Bottalico said.

Friends noted that Rockefeller, had recently gained as much as 30 pounds.

The NTSB will retrace the engineer’s 72 hours prior to the accident, which killed four and injured 63. Tests showed no alcohol in his system.

Immediately after Sunday’s wreck, Rockefeller told probers at the crash scene, “I don’t know. I was in a daze . . . I zoned out,’’ sources said

A law-enforcement source said Rockefeller’s description of the events was “akin to like driving a car, listening to your favorite song on the radio and zoning out on it and then all of sudden, you say, ‘Oh, s–t, I better slow down.’ ’’

The Bronx District Attorney’s Office and the NYPD have launched a criminal investigation.

Rockefeller — a 20-year MTA veteran with a clean work record — could face charges of involuntary manslaughter and reckless-endangerment, sources said.

Meanwhile, late Tuesday night, the NTSB removed the rail union from its investigation because of the comments that ACRE chief Bottalico had made to the press.

“While we value the technical expertise that groups like ACRE can provide during . . . an investigation, it is counterproductive when an organization breaches the party agreement,” said NTSB Chairman Deborah Hersman.

Also, Dr. Denise Williams, who suffered near-fatal spinal injuries in the wreck, became the first victim to file paperwork Tuesday to sue the MTA.

Additional reporting by Rebecca Harshbarger, Frank Rosario and Priscilla DeGregory