The engineer of the Metro-North train that derailed Sunday morning “nodded off” before he caught himself, a transit union representative said Tuesday.

New York's PIX 11 reports that union rep Anthony Bottalico said William Rockefeller “basically nodded.” But the National Transportation Safety Board, which is investigating the accident, stressed that it is too early to determine if the engineer was conscious before the derailment that killed four passengers and injured 63.   

“The sun hits you too much, you nod out and you catch yourself,” said Bottalico, head of the Association of Commuter Rail Employees, in trying to explain what happened. “This is a terrible tragedy, that’s what it is.”

Bottalico also added that Rockefeller’s work schedule had changed from mornings to nights on Nov. 17. The NTSB said nothing was wrong with the Metro-North train’s brakes or systems and breath tests of the crew have all returned negative for alcohol, reports CNN. But additional toxicology results are not in yet.

Speaking during a press conference, NTSB member Earl Weener said the train had a “dead man’s pedal” that is designed to halt the train if its engineer is impaired. But currently it is unknown if the system activated ahead of the derailment. As part of the investigation, the driver’s cell phone is also being checked.

After an initial analysis of the two "event recorders" recovered from the crash, the seven-car train was traveling at 60 mph two minutes before the crash. It then inexplicably increased in speed to 82 mph as it went into the curve, when it should have slowed to 30, said Weener.

The train’s operator is now on unpaid leave, an MYA spokesperson told ABC News. And according to a Twitter update on the incident by ABC News reporter Josh Margolin, the lawyer of train passenger Denise Williams has filed a legal claim against Metro-North.