After finding that the engineers in two recent commuter train crashes in the New York metropolitan area suffered from sleep apnea, federal investigators questioned the Trump administration’s reversal on requiring tests for the disorder.

At a hearing in Washington on Tuesday, the chairman of the National Transportation Safety Board said he was “mystified” by the withdrawal last year of a proposed federal rule on screening train crews for sleep apnea. The safety board officially concluded that the engineers’ fatigue and their employers’ failure to screen for sleep apnea caused the two similar crashes — one in Hoboken, New Jersey, in late 2016 that killed one person and another in Brooklyn three months later.

“The public deserves alert operators,” said the chairman, Robert L. Sumwalt. “That’s not too much to ask.”

Sumwalt, who led the three-member board through the hearing in Washington, had just returned from the scene of Sunday’s Amtrak crash in South Carolina that killed an engineer and a conductor. The cause of that crash appeared to be the unexpected diversion of the Amtrak train onto a a portion of track known as siding, where it ran headlong into a parked freight train.

The crashes in Hoboken and Brooklyn — both involving trains that ran off the end of a track — were just two of many examples of crashes the board investigated that involved driver fatigue, Dr. Nicholas Webster, the board’s medical officer, said. Screening crew members for obstructive sleep apnea, which disrupts breathing during sleep and causes drowsiness, is simple and inexpensive, Webster said.

But many railroads have not routinely screened their workers, and the Federal Railroad Administration has not ordered them to do so. The railroad administration had proposed mandating screenings for sleep apnea but withdrew that proposal in August 2017, as part of the administration’s effort to reduce regulations.

The crash in Hoboken involved a New Jersey Transit train that accelerated as it approached the terminal. Carrying at least 250 passengers, it ran through a bumping post at the end of the track and collided with the wall of the waiting room, killing a bystander and causing about $6 million in property damage.

After the crash, the engineer was diagnosed with obstructive sleep apnea, Webster said. He had been screened in 2013 and was found to have risk factors for the disorder, which include obesity, but he was not referred for further study, Webster said. The engineer weighed 322 pounds at the time of the deadly crash in late September 2016 and had gained 90 pounds in three years, he said.

Webster said New Jersey Transit was unable to locate the results of the engineer’s most recent screening. But he said that, since the crash, New Jersey Transit had screened all its crew members for sleep apnea and had taken those who required treatment out of service. The engineer involved in the Hoboken crash has not been allowed to return to work.

Sumwalt applauded New Jersey Transit for that decision, saying, “I know it is not popular, but it is the right thing to do.”

Nancy Snyder, a spokeswoman for New Jersey Transit, said the agency was “pleased that the NTSB acknowledged our aggressive sleep apnea screening protocol” and said the railroad was “developing a technology solution to enforce civil speeds at terminal stations.”

The other crash the board considered involved a Long Island Rail Road train that ran through a post at the end of a track at the Atlantic Terminal in January 2017. Nobody was killed, but more than 100 people were injured, and the accident caused more than $5 million in property damage.

In that crash, investigators determined that the engineer fell asleep as he was bringing the train into the terminal. The Long Island Rail Road had not screened its drivers for sleep apnea before the crash but has begun doing so, Webster said.

A spokesman for the Metropolitan Transportation Authority, which operates the Long Island Rail Road, said in a statement, “The MTA has an established and aggressive sleep apnea screening and treatment program for all train and bus operators and locomotive engineers in line with the NTSB’s recommendations and we are moving forward with this program, even in the absence of a federal mandate.”

After the Hoboken crash, New Jersey Transit began having a conductor move into the driving cab alongside the engineer as a train enters a terminal. Long Island Rail Road had been scheduled to adopt that procedure the day of the crash in Brooklyn, one of the investigators, Ted Turpin, told the board.

But Turpin said there was no data to show that having two people in the cab was any safer. “We have accidents with two people in the cab all the time,” he said.

Indeed, in the Amtrak crash in South Carolina on Sunday, the engineer and conductor who died were both in the front of the train, according to the safety board.

The safety board also determined that even with their engineers asleep or not fully alert, both of the trains that crashed could have been stopped by an automatic-braking system known as Positive Train Control. The Federal Railroad Administration has ordered railroads to have the technology installed and operating by the end of this year.

But New Jersey Transit and the Long Island Rail Road have received waivers from that mandate for the terminals where the crashes occurred. Positive Train Control, which uses satellites to locate trains, was designed to slow speeding trains on mainline tracks. Installing it inside terminals, where many tracks come together, often beneath roofs or underground, is difficult.

Still, the safety board concluded that “the lack of either a device or a safety system that could have intervened to stop the train before the collision” was a factor in each of the crashes. The board recommended a requirement that intercity railroads implement technology that will stop a train before it reaches the end of a track.

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