The National Transportation Safety Board has dropped the union representing rail workers as a participant in the probe into Sunday’s Metro-North derailment that killed four people in the Bronx.

The move came after the union's chief publicly disclosed details about the engineer who drove the train at a high rate of speed.

The federal investigative agency usually seeks technical expertise from a variety of industry, government and union sources as it conducts examinations into the causes of a deadly disaster. As part of the process, the outside parties agree to maintain confidentiality during the investigation.

But Anthony Bottalico, general chairman of the union, the Association of Commuter Rail Employees, told reporters on Tuesday that he believed that the engineer of the Manhattan-bound commuter train had nodded off moments before the train entered a curve and veered off the tracks.

Hours after his news conference, the NTSB announced that it had removed the union from the probe.

“While we value the technical expertise that groups like ACRE can provide during the course of an investigation, it is counterproductive when an organization breaches the party agreement and publically interprets or comments on investigation information,” NTSB Chair Deborah A.P. Hersman said in a prepared statement. “Our rules exist to avoid the prospect of any party to an NTSB investigation offering its slant on the circumstances of the accident.”

At the news conference, Bottalico said the train’s engineer,  William Rockefeller, had dozed off moments before the derailment at a curved portion of track near the Spuyten Duyvil station on Metro-North's Hudson Line. The NTSB has said the train was going 82 mph in a 30 mph zone.

“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,” Bottalico said. “How long that lasts, I can’t answer that. Only Billy can.

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

Earlier Tuesday, Jeffrey Chartier, an attorney for Rockefeller, said his client experienced  “a daze,” almost like road fatigue or the phenomenon sometimes called highway hypnosis. He couldn't say how long it lasted. Chartier spoke to reporters after going with Rockefeller to an interview with investigators.

“You've got a good guy and an accident,” Chartier said. “A terrible accident is what it is.”

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