The White House has pulled an Obama-era proposal that would have mandated two crew members in most locomotives, and it has banned states from requiring railroads to do so.

The proposal had been stalled by the Office of Management and Budget under President Barack Obama and by top Department of Transportation officials, who said there was no evidence that two-member crews made trains safer.

A former senior-level Obama official, who spoke on the condition of anonymity because he did not want to jeopardize his current job, conceded that point.

“The challenge that we had was that we had anecdotal evidence” suggesting two-member crews would improve performance, the official said, “but we didn’t have empirical evidence that it would make it safer.”

The Federal Railroad Administration decided to withdraw the proposed rule because “no regulation of train crew staffing is necessary or appropriate for railroad operations to be conducted safely.”

The FRA said it would not allow individual states to regulate staffing of train crews. At least nine states have approved laws that mandate crew size, the agency said.

In an era when autonomous cars are seen on the horizon, advances in railroad technology have made the prospect of automatically run trains, if not yet on the horizon, perhaps just over it.

For example, positive train control, with onboard computers and trackside sensors, is expected to virtually eliminate human error when it is fully implemented by the end of 2020.

Although they are far less complex, rail shuttles between airport terminals already operate without crews on board.

“The railroads losing their minds over this proposal wasn’t about current operations,” said the former Obama official. “It was always about the ability to go to fewer [crew members] in the future. What [the proposed rule] was saying is that if you want to go to fewer than two crew members on a train, that the Federal Railroad Administration and the American public should have a seat at the table to verify that all safety measures were being taken.”

Most trains run with two crew members: an engineer and a conductor. On passenger trains, often with more than one conductor, the chief conductor is in the passenger cars, but remains in communication with the engineer. On freight locomotives, however, the conductor may be in the locomotive unless he or she has more pressing duties.

Labor unions said the FRA decision to pull the rule “placed corporate profits above public safety.” In a joint statement, two unions cautioned against autonomous trains.

“If the ongoing grounding of the Boeing Max aircraft has taught us nothing else, FRA and the Department of Transportation should be mindful of the danger of transferring the risk of a human-factors accident from [crew members] to [a computer] programmer when autonomous technology is implemented.”

John Previsich, president of the Transportation Division of the International Association of Sheet Metal, Air, Rail and Transportation Workers, elaborated on the joint statement.

“The threat is that, in the face of automation, as we’re seeing in just about every industry, the financial pressure on the railroads is making them desirous of one [crew member] or none,” Previsich said. “They want no crew regulations period. The FRA said not only are we going to drop it, but we don’t believe that we need to oversee automation at all, which is astonishing because there’s no transportation mode in the country that doesn’t have some regulatory oversight for automation.”

Warren Flatau, an FRA spokesman, said pushing a new rule into place was “an unnecessary obstacle to future innovation in the rail industry.”

Flatau said railroads have “a strong safety record in the absence of regulation on this issue and that regulating train crew staffing is not necessary or appropriate.”

Previsich disagreed.

“We believe that the recent actions of FRA pose a significant threat to the safety of our members and the public,” he said.

The rulemaking process was invoked by then-FRA Administrator Joseph C. Szabo after an unattended 74-car freight train carrying crude oil ran downhill and crashed in the Canadian town of Lac-Mégantic, killing 49 people in 2013.

The explosion destroyed most of the town, leaving just three downtown buildings standing.

Later that year — still on Szabo’s watch — two trains collided in Casselton, N.D., causing several explosions of 476,000 gallons of crude oil and a massive cloud of smoke that led to the evacuation of the surrounding area.

Although no one died, Casselton’s mayor said the city had “dodged a bullet,” and lawmakers were placated by Szabo’s promise to require two-man crews.

In deciding to revoke the proposed rule this year, the FRA concluded, “There is no direct safety connection between train crew staffing and the Lac-Megantic or Casselton accidents.”

In addition to an engineer and conductor, Amtrak said it uses a second engineer on routes that take six hours or longer.