Does anyone remember how to behave on the subway?

There are unwritten rules when traveling on the New York City subway.

No eye contact. No blocking doors. No high chairs.

Three years after the coronavirus nearly wiped out ridership on the nation’s largest transportation network, millions of travelers are reacquainting themselves with the casual ways of public transportation. The pandemic had suspended these protocols as many passengers stayed home. But now, in a sign that normalcy may be returning, old tensions are creeping back on the subway as more passengers enter the system.

“We’ve lost a little bit of that muscle mass,” said Khalid Ahmed, 35, a data scientist from Brooklyn who drove into Manhattan last month to meet friends. “It is now a bit more surprising when the trains are packed like sardines. That was a fact four years ago.”

New Yorkers must follow a long list of norms and practices to manage the hustle and bustle of people they interact with every day, says Jay Van Bavel, a professor at NYU who specializes in social psychology.

“It’s probably the place in New York that pushes you against humanity more than any other place,” Mr. Van Bavel said. ‘It would be insane if I pressed myself against a stranger’s body on the street for ten minutes, wouldn’t it? For example, they would panic. But how many times have you been on the subway and people have been pressed against you?”

Although public transportation, operated by the Metropolitan Transportation Authority, is not yet operating at full capacity, it is still used by about four million people every weekday — a total that exceeds the population of some states. And if riders do not follow the rules, officially or not, emotions can run high.

During the morning rush hour at Grand Central Terminal last month, one woman cursed at another as she lingered on a staircase. That afternoon, two men at the Times Square station got into an argument in Spanish after one of them complained about people blocking his way.

Huma Khalid, 30, a bank teller from Brooklyn, said she is frustrated by loud and rude riders. A woman sitting next to her shouted expletives and insults at fellow passengers during a recent train ride to Manhattan. Mrs. Khalid rolled her eyes as she looked away.

“I can’t take it anymore,” Ms. Khalid said. “This is terrible.”

The woman, who declined to give her name and did not explain why she was annoyed, said she had just moved to New York from Atlanta.

While everyone has individual pet peeves, many seem to agree on the major, unwritten offenses. For example, riders should never spread their arms or legs, as this can jostle or trip fellow passengers.

“Sometimes there are people lying down, and everywhere there are people standing,” Ms. Khalid said.

And then there is the traffic jam when getting on and off the train.

“Let people get off the train first to make the commute easier,” said Maykon Reyes, 30, a security guard who lives and works in the Bronx.

Another common source of frustration for New Yorkers is people playing loud music through their cell phones or Bluetooth speakers. The noise is not only annoying, but can also drown out subway announcements.

Those riding while standing should be careful not to hold a pole in a way that prevents others from grabbing it. Transit passengers get especially upset about fellow passengers leaning on shared poles.

And no matter how full the train is, no one should ever come near a stranger in a way that makes him or her uncomfortable.

Other major blunders include placing backpacks or handbags on empty seats, cramming bikes and scooters into crowded cars, and eating smelly food.

Mr Van Bavel says these core violations are particularly frustrating because they show a lack of respect for fellow riders.

“It shows you’re not a nice person,” Mr. Van Bavel said. “You are not fair and you welcome or create chaos.”

But the pandemic has eroded norms of behavior. At the height of the crisis, many commuters avoided the system and fell out of practice. The coronavirus also brought new rules, such as social distancing and a greater need to protect others from coughing and sneezing.

At the same time, recent arrivals have not as easily assimilated as previous waves of people because there have been fewer New Yorkers to model their behavior on. And when people interact in places with uncertain norms, they can experience anxiety and conflict.

Surveys among riders show that while customer satisfaction has improved overall, it has declined and even declined in recent months 58 percent of customers satisfied with the metro in September. The MTA’s stated goal is 70 percent system satisfaction by June of next year. In the latest survey results, many riders said they would come back if fewer people behaved erratically in the system.

Still, some New Yorkers said the pandemic has brought benefits. Pat Golden, a film and theater director who lives and works in Manhattan, said she enjoyed the quiet while traveling on sparsely crowded trains. Ms Golden said people seem to be more conscious these days about not sitting too close to each other.

Jennifer Meeker, 53, a digital producer who lives on Long Island and works in Manhattan, said people also seem more vigilant.

“It’s changed a little bit as far as common pleasantries,” Ms. Meeker said. “I’ve noticed a little bit of a difference in the general population being a little more protective of each other, which I think is nice.”

Of course, there are official transit rules, and some of them are punishable by law. Fare evasion, smoking and lying on more than one seat can all lead to police action. To make passengers feel safer, transit officials have focused on issuing citations increased by 55 percent from January to September compared to the same period last year.

On September 20, the metros recorded approximately 4,180,000 paid trips, a new daily high since the pandemic began. These figures have remained largely stable. In response to rising ridership, the MTA is also trying to make the system more inviting new advertisements encouraging riders to be polite.

The signs, launched last month, feature cartoon drawings that emphasize good behavior such as not littering or applying makeup.

“When we heard feedback from New Yorkers, we summarized a lot of the things that drive people crazy,” said Shanifah Rieara, who oversees rider satisfaction for the authority. She added that vaping on trains has become a problem.

The authority has been continuously launching such marketing campaigns. An earlier version in 2014, riders called for clipping their nails, carrying backpacks and manspreading – a widely detested sitting style where you open your legs in a V-shape and crowd others into nearby chairs. In 2017, the authority issued pregnant women with blue and yellow buttons with a message asking fellow passengers to offer them a seat.

The latest campaign comes as the MTA is under pressure to improve service and win back riders, in part because the state mandated it as part of a budget deal. Transit advocates have said the authority should focus on communicating effectively with passengers to keep them happy, and its new marketing campaign aims to do just that.

Perhaps not surprisingly, some New Yorkers were quick to reject the idea.

“I think it’s foolish,” said Mr. Reyes, the security guard. “People don’t respect the rules.”