An undiagnosed sleep disorder may have led to the engineer falling asleep on a train involved in a deadly New York City derailment, the U.S. National Transportation Safety Board says.

Four people died and at least 61 people were hurt when a commuter train bound for Manhattan derailed as it traveled around a curve on Dec. 1, 2013.

At the time, the train was traveling at least 50 mph faster than the 30 mph speed limit for that section of the line, the NTSB said.

In its report, the safety board said the train's engineer, William Rockefeller, suffered from severe obstructive sleep apnea that had not been diagnosed.

In the sleep disorder, a person's airway collapses and obstructs normal breathing periodically, resulting in poor sleep and often leading to drowsiness or difficulty in concentrating during the day.

Rockefeller told NTSB investigators he was overcome by a "hypnotic" state that kept him from realizing the train was in danger of derailing.

Sleep apnea has also been linked to high blood pressure and elevated risk of diabetes, heart attack and stroke.

The most noticeable symptom of possible obstructive sleep apnea is snoring, experts say.

It is most commonly seen in middle-aged or older adults, and is often linked with being overweight.

Rockefeller's condition was magnified by a change to his work schedule, in which he was transferred to an early-morning shift around 2 weeks before the accident.

The incident, in which the locomotive and all seven cars derailed, was the first fatal accident in the 31-year history of the Metro-North Railroad.

In addition to findings about the engineer, the NTSB said there was evidence that pressure on railroad supervisors to maintain train schedules resulted in skipped inspections and maintenance.

"A mistake is a tragedy when you don't learn from it," the company's president Joseph Giulietti said at a briefing at Manhattan's Grand Central Terminal. The railroad has taken its lesson to heart and was committing itself to becoming "best in class," he said.

Since the accident the railroad has instituted a number of safety measures on the line, he said.

Those include upgraded engine controls with automatic speed control on curves, "alerter" devices to keep drivers responsive, and cameras in locomotive cabs that face both inward and outward.

While a sleep-disorder screening program has voluntarily been put in place by Metro-North, there are currently no federal regulations requiring medical screening to detect such disorders, the NTSB said.