Subway passengers were left stranded and broiling on train platforms for more than an hour on Friday night on the first day of a heat wave as several lines in New York City were suspended in both directions.

The Metropolitan Transportation Authority attributed the disruption to a “network communications” issue, and said on Twitter just before 6 p.m. the Nos. 1, 2, 3, 4, 5 and 6 lines and the Times Square shuttle were all affected.

About 90 minutes later, it was reporting that service was slowly being restored. But by that point, passengers and politicians alike had been fuming over the abrupt interruption in service during the evening rush.

The failure, critics said, underscored the problems affecting an aging, troubled mass transit system. The widespread disruption was the second major infrastructure failure in the city in a week. A blackout last Saturday had cut power to the subways, forcing passengers to be evacuated.

At a news conference on Friday night, Andy Byford, president of New York City Transit, an arm of the M.T.A. that runs the subway and buses, said the root cause of the failure was not immediately clear.

“Our rail control center could not see where trains were on the numbered lines,” he said. “It was the servers that had failed such that the signals could not be seen.”

New York City officials were calling for answers.

Mayor Bill de Blasio said on Twitter: “This kind of meltdown during a heat wave is UNACCEPTABLE. The MTA owes every single New Yorker an explanation for this. We’ve known about this dangerous weather for DAYS. There’s no excuse for why they aren’t prepared.”

“This is completely unacceptable @MTA. Service is suspended and platforms are boiling. New York cannot function like this,” the city comptroller, Scott M. Stringer, said on Twitter.

Mr. Byford did not respond publicly to the criticism.

“I’m frustrated as well,” he said. “This could not have happened at a worse time. It was the height of the evening peak.”

Tim Minton, the M.T.A.’s communications director, said in a statement that “for safety reasons, trains were required to maintain their positions at the time of the interruption, and some of those trains were in between stations when that occurred.”

He said service was starting to be restored at 7:16 p.m., with substantial residual delays. Power for lighting and air-conditioning remained on during the disruption, Mr. Minton said. Officials said they did not believe the failure was heat- or power-related.

Danny Pearlstein, director of policy and communications for the Riders Alliance, said in a statement that breakdowns like the one on Friday would become more common until a modern signal system was installed.

“Tonight, the subway’s 1930s-era signals melted down as hundreds of thousands of transit riders waited in dangerous, sweltering heat,” the statement said. “The M.T.A. has made critical strides by speeding up trains to cut delays but only modern signals will deliver the reliable subway system New Yorkers need.”

On social media and in interviews, passengers described oppressively hot conditions while waiting for trains on platforms.

Maxine Wally, 29, a journalist for Women’s Wear Daily, said it took two and a half hours to go from Grand Central Station to her sister’s house in the Prospect Lefferts Gardens neighborhood of Brooklyn.

She said she waited 40 minutes for a No. 5 train at Borough Hall that was stuck inside the station before switching to a crowded bus.

“One guy got onto the bus and he was saying he was going to pass out from the heat,” Ms. Wally said. “He was cursing at people to move in. A fight almost broke out on the bus. It was madness — a real New York moment.”

Trevona Brown and her friend paused in a busy area at the 14th Street-Union Square station where the police had blocked off stairs to the Nos. 4, 5 and 6 trains with yellow tape.

A harried M.T.A. worker wearing an orange vest and a white towel around his neck raised his hands, trying to direct people to alternate options, but Ms. Brown was looking for suggestions from her phone instead.

“I’m just trying to figure out what’s going on,” she said, explaining that she was trying to get to SoHo. “I don’t really take the train much,” she said. “This is why.”

At Times Square, an M.T.A. employee in an orange vest guarded the 1-2-3 line and stopped passengers from taking the stairs. “No train, no train,” she shouted.

Kirk Polius, 28, was trying to get to his home in the Bronx from the Upper East Side neighborhood. He ended up at Times Square.

“I moved here almost two years ago from Houston and this is the most frustrating part of living here,” he said of the disruptions. “The transportation is never reliable. You either get stuck somewhere and have to transfer or you’re just sitting and waiting and waiting and waiting without any idea of what’s going on.”

Reporting was contributed by Nicholas Bogel-Burroughs, Emma Fitzsimmons, Michael Gold, Sarah Mervosh, Derek Norman, Andrea Salcedo, Neil Vigdor and Mihir Zaveri.

Christopher Mele, a Bronx native, is a senior staff editor. He joined The Times in 2014 on the Metro copy desk. For nearly 30 years before that, he worked as a reporter and editor at newspapers in the Adirondacks and Hudson Valley of New York and the Poconos in Pennsylvania. @MeleChristopher

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