A Metro rider takes a picture of the snow-covered Potomac River on Jan. 22, 2016, hours before the unprecedented shutdown of the system for the weekend due to a powerful snowstorm forecast to hit the Washington region. (Jonathan Ernst/Reuters)

Metro’s extraordinary decision to shut down mass transit in the Washington area during a potentially historic blizzard was driven mainly by safety concerns, the agency’s board chairman said Friday. But he said there was more to it than that.

If the nation’s second-busiest subway had stayed open for the convenience of a relatively small number of weekend riders, Chairman Mortimer L. Downey said, the odds of full service being available Monday would have been considerably reduced.

And with hundreds of thousands of commuters possibly returning to work Monday morning, depending on post-storm conditions, Metro chose the lesser of two evils.

“I know some people will be put in a difficult situation,” Downey said, referring to essential employees such as hospital and public-safety personnel, as well as others who must get to their jobs on weekends, including hotel workers.

“All we can tell them is, ‘We’re very sorry, but we had to make a decision for the good of the majority of our customers,’ ” he said, meaning the riders who make more than 700,000 passenger-trips on Metrorail each weekday.

Metro's General Manager and CEO Paul J. Wiedefeld announced Thursday that rail and bus systems will be closed this weekend due to the snowstorm heading toward the region. (WUSA9)

Closing the subway and sheltering rail cars in idle tunnels will make a massive snow-removal effort easier for Metro crews before Monday’s start of the work week, Downey said. The agency also suspended bus service for the weekend, as well as MetroAccess, the door-to-door van service for disabled riders.

The dilemma — an unusual one for Metro, but not a unique problem in the mass-transit industry — forced General Manager Paul J. Wiedefeld to make his biggest decision since taking over as the beleaguered agency’s top executive shortly after Thanksgiving.

In announcing the shutdown, Wiedefeld said that the snowfall, which could exceed two feet in parts of the region, would make above-ground train service virtually impossible during the storm. And he said high winds and resulting electrical outages might have stranded riders in tunnels if Metro had tried to operate trains only in underground parts of the system.

Like other major subways, Metrorail is too large to run entirely on self-generated power, said Downey, a career transportation official who was executive director of New York’s mass-­transit system in the 1980s. “I don’t know of any transit system that does that today,” he said. “They all have to rely heavily on outside utility companies.”

In Chicago, for example, a major power failure in the city would mean “limited service” in the subway, said Catherine Hosinski, a spokeswoman for the Chicago Transit Authority.

Still, she said, there is no record of the Chicago’s subway closing “proactively” in advance of adverse weather. Citing a litany of major snowstorms and arctic-style deep freezes that her city has endured, Hosinski said: “We operate 365 days a year, no matter what.”

New York, however, cannot say the same. At 11 p.m. last Jan. 27, with a blizzard thought to be bearing down on Gotham, the city’s subway was shut down ahead of a storm for the first time in its 110-year history, and hundreds of rail cars were sheltered in tunnels.

Then, in the wee hours of Jan. 28, it became clear that the blizzard would deliver only a glancing blow to New York. It was 9 a.m. before the subway began operating again.

This weekend, “we will make every effort to keep our services up and running,” Thomas F. Prendergast, chairman of New York’s Metropolitan Transportation Authority, said in a statement.

But “there may come a point throughout the winter months when it is no longer prudent to roll out buses or send trains onto some outdoor sections of the lines.”

As for inconveniencing would-be Metro riders Saturday and Sunday, perhaps prompting some to dare driving to work in hazardous conditions, Wiedefeld skirted the issue at a Thursday news conference. He simply reiterated that officials throughout the region were urging people to stay home during the storm.

“It was absolutely the right call,” Downey said of Wiedefeld’s decision. The weekend closure is believed to be the longest in Metro’s 40-year history. During Hurricane Sandy in October 2012, the system closed for about 36 hours.

“It’s the call I would have made,” Downey said.

After one of the most troubled periods in Metro’s history — including repeated, sometimes calamitous service breakdowns in 2015 and unremitting criticism of the agency by riders and public officials — Wiedefeld also was clearly eager to expedite weekend snow removal and maximize the chances of avoiding a post-storm commuting debacle.

By sheltering about 900 rail cars in tunnels and allowing workers unfettered access to the idle tracks, he said, crews can continue plowing throughout the weekend, keeping up with the snowfall, and will not have to shovel out hundreds of snowbound rail cars after the storm, which might not end until Sunday.

Of Metrorail’s 117 route-miles (there are 234 miles of actual tracks, running in two directions), about 60 are above ground.

However, there were no promises about a Monday reopening. “Let’s see how the storm plays out and what we have on our hands,” Wiedefeld said.

In Boston, in the early weeks of 2015 — as one giant storm after another walloped the Northeast — the Massachusetts Bay Transportation Authority shut down its oldest-in-the-nation transit system twice, each time for more than 24 hours. The MBTA’s general manager, Beverly Scott, resigned in February, complaining that the system had long been starved of money, technology and other resources.

“I’ve seen cases in the past where transit managers say, ‘We’ve got a tough system’ and ‘We have to service the public — we’re going to keep running,’ ” Downey said. “And then they totally lose control during the storm. And they live to regret it.”

Craig DeAtley, the head of emergency management at MedStar Washington Hospital Center, said Metro’s decision to close came as a surprise. But the Thursday announcement gave the hospital enough to time get ready by bringing in weekend staffers early and preparing sleeping quarters for them to stay until Monday.

“I have been around for 43 years and I don’t remember Metro ever closing the whole system for two days,” DeAtley said. “But, candidly, I don’t consider that to be a bad idea. Nobody should be out and about” in the storm.

However, he said, “We will be anxious . . . for Metro to resume on Monday.”

Meanwhile, the Maryland Transit Administration canceled MARC commuter rail service Saturday on the Penn Line.

Officials said they will decide Saturday night whether the line will operate Sunday, and will decide later about Monday service. The region’s other major commuter railroad, Virginia Railway Express, does not operate on weekends.

“We will be ready for service on Monday,” VRE spokesman Bryan Jungwirth said.

In Philadelphia, the Southeast Pennsylvania Transportation Authority also suspended weekend commuter rail service. But the Philadelphia subway’s two major lines, also operated by SEPTA, will stay open, General Manager Jeffrey Knueppel said in a statement.

“The Broad Street and Market-Frankford lines are important to carry emergency personnel, hospital employees, and other essential workers during the storm,” Knueppel said.

“We also have the option of continuing service on these routes just within the underground tunnels should operating conditions become unsafe above ground.”

Amtrak spokeswoman Chelsea Kopta said the railroad intends to keep operating along the Northeast Corridor line this weekend, including service in and out of Washington. But she had no advice for passengers who arrive at Union Station with plans to ride Metro’s Red Line from there to their final destinations.

“We know a lot of people are wondering what is going to happen with public transportation this weekend,” she said. “Like the weather, this is unpredictable. All we can control is Amtrak, and we are doing the best we can.”