A sleep-deprived engineer nodded off at the controls of a commuter train just before taking a 30 mph curve at 82 mph, causing a derailment last year that killed four people and injured more than 70, federal regulators said Tuesday.

William Rockefeller’s sleepiness was due to a combination of an undiagnosed disorder — sleep apnea — and a drastic shift in his work schedule, the National Transportation Board said. It said the railroad lacked a policy to screen engineers for sleep disorders, which also contributed to the Dec. 1 crash. It also said a system that would have automatically applied the brakes would have prevented the crash.

The board also issued rulings on four other Metro-North accidents that occurred in New York and Connecticut in 2013 and 2014, repeatedly finding fault with the railroad.

“This would be almost a comedy of errors if it weren’t so tragic,” Sen. Chuck Schumer, D-N.Y., said Tuesday. “It’s clear these mistakes were avoidable.”

The NTSB had reported Rockefeller’s sleep apnea in April, saying tests revealed it interrupted his sleep dozens of times each night. Investigators said Rockefeller told them he had felt strangely “dazed” right before the crash. But until Tuesday it had refrained from declaring his sleepiness the cause of the crash.

It said that less than two weeks before the crash, Rockefeller had switched from a work day that began in late afternoon to one that began early in the morning. The board said that probably compounded his sleep problem.

It also noted that the technology known as positive train control was not in use at the time of the crash. Positive train control can automatically bring a train to a stop if it’s exceeding a speed limit. Metro-North has said it is working to install the technology.

On the other accidents, the NTSB found:

—A May 17, 2013, derailment and collision in Bridgeport, Connecticut, was caused by broken joint bars, which are used to join rails of different sizes. At least 65 people were injured. The board said Metro-North had deferred scheduled track maintenance and lacked “a comprehensive track maintenance program.”

—A track foreman who was fatally struck by a train in West Haven, Connecticut, on May 28, 2013, was probably due to a mistake by a student rail traffic controller. The controller misunderstood some instructions and canceled the signals protecting the section of track the man was on, the NTSB said.

—In a similar accident in Manhattan on March 10, 2014, a worker was killed by a train while trying to re-energize tracks that had been out of service for maintenance. The NTSB blamed the accident of briefings that poorly communicated which part of the track would be safe.

—The derailment of a freight train on Metro-North tracks in the Bronx on July 18, 2013, which caused no injuries, was caused by deteriorated concrete ties and other problems compounded by deferred maintenance, the NTSB said.

In March, the Federal Railroad Administration issued a stinging report on Metro-North, saying the railroad let safety concerns slip while pushing to keep trains on time. Railroad executives pledged to make safety their top priority.

Sen. Richard Blumenthal, D-Conn., said Monday that the new NTSB report documents “the cascading catastrophes over a single year illustrating the urgent need for dramatic upgrades and improvements in safety and reliability.”

Metro-North is the second-largest commuter rail line in the country. It carried more than 83.4 million riders between New York City and its suburbs last year.