Zackery Nazario’s bedroom in the East Village is full of reminders of the 15-year-old who lived there.

One afternoon this summer, a Derek Jeter jersey in a kid’s size medium hung from a hook on the wall. Zackery’s mother, Norma Nazario, made her way around the room, rattling off other things her son loved: Slurpees from 7-Eleven, his cat, history trivia, any Christmas song by Frank Sinatra.

She smiled at a picture of Zackery’s last day of kindergarten, when he had insisted on slicking his hair into a spiky Mohawk. “I wasted two bottles of the real thick gel to get it to stay like that,” she said.

The bedroom has been empty since Feb. 20, 2023, when Zackery was killed while riding on top of a subway car as it crossed the Williamsburg Bridge. Another thing Zackery loved was subway surfing, a deadly activity in which thrill-seeking teenagers scale moving trains.

At 6:45 p.m., Zackery climbed between cars of a Brooklyn-bound J train and was struck in the head by a low beam while turning around to look at his girlfriend, according to court filings. He fell between subway cars and was run over by the train.

Zackery died while doing something that he had been posting about on social media for months. On Instagram, he’d documented himself clinging to the back of a train cab as it sped through the Prospect Avenue stop in Brooklyn, and striking a swaggering pose for the camera while standing on top of a J train that was approaching the Williamsburg Bridge.

He tagged friends who were part of the same online community as him, one in which teenagers posted short films that in Hollywood would require professional stunt performers. The exercise can be lethal: Seven people died last year while riding on the outside of trains, according to the N.Y.P.D. The youngest was 11.

TikTok and Instagram say that subway surfing videos are not permitted on their platforms. But they spread there anyway. The New York Times viewed dozens of these videos online, filmed above and underground, in broad daylight and at sunset. Some disappeared after a day or two, but others stuck around, including a video on Instagram of a young man in a construction vest doing a jumping jack on top of a moving train. On TikTok, a montage of shaky train surfing clips, set to “Dancing Queen” by ABBA, has been up for nearly two years.

In interviews, teenagers who inhabit this world say they are attracted to the adrenaline rush, the social media likes and the age-old desire to get away with something off limits. Once they start, they say little else gives them the same stomach-churning thrill.

Ms. Nazario, 54, a nurse who came to New York from Puerto Rico as a child, said she did not know her son had been subway surfing until it killed him. On the day he died, Zackery told his mother that he was going for a bike ride with his girlfriend. Ms. Nazario suggested they all meet up later to eat at P.F. Chang’s.

That evening, when he still had not returned home, she called him, texted him and called him again. Around 10 p.m., two detectives arrived at her door and sat her down on the couch, she said. One took her hand.

“He was doing what?” Ms. Nazario remembered saying.

Later that week, she went to the police precinct in Union Square to retrieve Zackery’s belongings. She took his chain necklace and his cross-body Nike bag, covered in winking emoticons, that had been mangled in the accident.

She was also given his iPhone, which was perfectly intact.

Zackery’s phone filled in the gaps for Ms. Nazario, exposing her to a vast ecosystem of subway surfing content on social media that she believes ensnared her son. She had thought that social media was mostly a place where teenagers kept in touch with their friends. Once she immersed herself in her son’s digital world, that changed. In his phone, she saw a posthumous message from her son about how she should channel her grief.

Last year, she filed a lawsuit in New York against the owners of TikTok and Instagram, arguing that their products had “targeted, goaded and encouraged” Zackery to engage in a life-threatening activity. She scored a preliminary victory in June when the judge allowed the case to proceed to discovery, over the objections of lawyers for the social media defendants. (Ms. Nazario also brought claims against the Metropolitan Transportation Authority, which were dismissed.)

TikTok declined to comment on Ms. Nazario’s case specifically, but said it had a trust and safety team of tens of thousands of employees, and that a vast majority of videos taken down for violating its dangerous activities and challenges policy were removed proactively, before they received any views.

A spokesman for Meta wrote in an email that the company was “disappointed” by the ruling, but that it had done nothing to address the merits of the case.

“Leaders and transportation authorities have grappled with the challenges of subway surfing for decades,” the Meta spokesman said. “Videos encouraging this kind of dangerous activity violate our policies, and we remove them when we become aware of them.”

To Ms. Nazario, those kinds of responses sound like an abdication of responsibility. She now moves through life with a four-word mission.

“To stop this nonsense,” she said.

‘Insanely Dangerous’

The door to Zackery’s bedroom is dark brown wood, with “I miss u” scrawled at eye level in black marker. In pink, above the doorknob, “I’ll see you on the other side brotherman.”

Ms. Nazario let Zackery’s friends come over and write the farewell messages after the funeral. To friends, it’s hard to wrap their minds around the fact that Zackery is really gone. In interviews, they described him as an outgoing, curious teenager who was often on the hunt for excitement.

He was born in the Park Slope neighborhood of Brooklyn and settled with Ms. Nazario in a public housing complex in Manhattan on the East River. He had a half brother who was 19 years older; Zackery’s father is not involved in the lawsuit and was not a part of his life when he died, Ms. Nazario said.

Zackery got a phone when he was around 12, according to court filings. His mother said he used it to send Snapchat messages to friends and play Subway Surfers, a mobile game in which an animated character collects coins by leaping atop train cars.

Ms. Nazario said she did her best to supervise his social media use, at one point sharing an Instagram account with him. But as a teenager, Zackery grew so obsessed with his phone that she would take it away and hide it. He would track it down. Eventually, he made a new Instagram account that Ms. Nazario did not know about.

During the pandemic, Zackery grew fascinated by the urban exploration community on social media, said Alex Cutting, 18, one of his friends and classmates at The Clinton School, a public school in Manhattan. Urban explorers — urbexers, for short — explore abandoned factories or rail tunnels and document their discoveries online.

Alex knew that Zackery would sometimes visit abandoned train tracks in the Bronx, but he was alarmed when his friend began posting pictures of himself subway surfing on Instagram in the fall of 2022. When Alex cautioned him that the activity was “insanely dangerous,” Zackery responded that subway surfing brought an unbeatable rush — one that made him feel like Matthew McConaughey in an action movie.

“He was like, ‘You know the scene in “Interstellar” where he goes through the black hole and everything shakes?’” Alex recalled. “‘That’s what it’s like to ride hanging off the back of the L train, all the way under the East River into Brooklyn.’”

In the last year of Zackery’s life, he told friends he had been feeling down and was having relationship trouble. But he still had goals for the future, including graduating from high school and joining the Marines.

“He wasn’t a bad kid — he wasn’t a troublemaker who wanted to die,” said Bek Metaliaj, 18, another friend from high school.

Not long before Zackery died, the two argued in a bathroom stall about subway surfing, Bek said. Zackery was spending more time with a group of friends who surfed regularly, and Bek was worried about his safety. “He said, ‘I know what I’m doing, I haven’t made a mistake yet,’” Bek recalled.

This is not an uncommon sentiment among subway surfers: that they know their way around the subway system better than anyone, and that this will keep them safe.

Terrell Ismail was 14 and a budding videographer when he first climbed on top of a moving train. He, too, had started out with urban exploration before getting into subway surfing. He discovered that posting videos on TikTok and Instagram of the activity could get him hundreds of thousands of views.

“I would see it online, and think in my head, ‘Oh that’s beautiful,’” he said in an interview. “‘That’s a neat way of getting a scenic view.’”

Just after his 15th birthday, Terrell was on top of a Manhattan-bound 7 train when he collided with an overpass in Queens. He sustained a traumatic brain injury that left him without the function of the left side of his body, said his father, Sammy Ismail. The family has been searching for a physical therapist who can come to their home, giving Terrell’s parents more time to make money to pay for his care.

Terrell, now 18, said he still scrolls past videos of people subway surfing on social media. “I comment, like, ‘Oh you shouldn’t do that,’” he said. “‘That’s going to ruin your life.’”

What Would Make Teens Stop?

The announcement, played periodically over the tinny subway speakers, is clear: “Ride inside, stay alive.”

It is part of a revamped public information campaign that Gov. Kathy Hochul of New York rolled out this summer with the M.T.A. and other agencies to warn of the dangers of subway surfing. The announcements have not dissuaded Angel, a 15-year-old in Queens, who asked that his last name be withheld to discuss an illegal activity.

He sometimes uses the P.S.A. slogan as a caption for videos of himself subway surfing. He and his friends hear the announcements all the time, he said. They laugh at them.

Beyond the announcements, the M.T.A. said it reports subway surfing videos to social media platforms, and that 1,800 had been taken down so far this year. Since 2023 the N.Y.P.D. has been flying drone cameras above hot-spot subway stations in order to intercept surfers.

More than 100 people have been removed this year while surfing, said Kaz Daughtry, the city’s deputy mayor for public safety. Each time he gets a call that another teenager has died subway surfing, “I feel a little angry and annoyed,” he said. “I feel like we really let that family down.”

Accounts of subway surfing date back to the earliest days of the city’s transit system. But the death toll has increased in recent years, said Joseph M. Gulotta, the chief of transit for the N.Y.P.D., a surge he and other officials attributed to social media. Last year, more than two dozen New York state lawmakers sent a letter to Meta, YouTube, Snapchat and TikTok saying that the companies had not lived up to their promises to remove subway surfing content.

Zellnor Myrie, a state senator from Brooklyn, was motivated to send the letter after an Instagram Reel of a person riding on top of a train appeared in his feed. “If I could, as a geriatric millennial, get this to my phone, this is certainly going to our young people,” he said in an interview.

The companies say the videos are not allowed and searches for “subway surfing” on both TikTok and Instagram lead to safety warnings, and no results. Still, as Ms. Nazario discovered, the videos are not difficult to find.

‘No One Is Stepping Up’

When Ms. Nazario opened Zackery’s phone, she said she saw videos from inside subway tunnels, footage of abandoned train stations and snapshots of Zackery riding the back and the top of trains. It was “mind-blowing,” she said.

Court filings include screenshots of posts that Ms. Nazario’s lawyers say surfaced in the feed that Zackery was shown on Instagram. They include a photo that appears to have been taken by someone on the back of an underground train cab (caption: “Midnight run”) and an image of a man leaping across the tracks, a train in the background.

Ms. Nazario reached out to the lawyer Matthew Bergman after seeing him on a “60 Minutes” segment alongside the Spence family, clients of his who sued Meta arguing that Instagram had led their 12-year-old daughter to be hospitalized for depression and an eating disorder. (The case is ongoing.)

“You hear this all the time — well, isn’t it the parent’s responsibility?” Mr. Bergman, a former asbestos lawyer who turned to suing tech companies in 2021, said in an interview. He argues that even the most clued-in of parents face an uphill battle against algorithmic feeds designed to retain their children’s attention by practically any means necessary.

“We are up front: Zackery made a very, very bad decision,” Mr. Bergman said. “Teenagers make bad decisions — they don’t deserve to die for it.”

Still, legal experts say that these kinds of cases typically do not succeed because of Section 230 of the Communications Decency Act of 1996, a federal law that has since the early days of the internet protected online platforms from being held liable for things that their users post.

Others see an opening. In the past five years, courts have demonstrated a willingness to read that protection more narrowly, said Alan Z. Rozenshtein, an associate professor at the University of Minnesota Law School. These cases are unpredictable, he said, but he does not think that winning is out of the question for Ms. Nazario.

With enough teenage casualties, “it just becomes very hard — rightly so, I think — for judges to say, ‘Well that’s just the cost of having a vibrant internet,’” he said. If a jury decides in favor of Ms. Nazario, he added, “the implications for the trillion-dollar social media industry are enormous.”

What a legal victory would not do is bring back Ms. Nazario’s son. His cat, Luna, a tabby with piercing green eyes, still curls up in Zackery’s bedroom, waiting for the teenager who used to dote on her.

And Ms. Nazario keeps telling one legislator after another about the dangers of subway surfing, all in the hopes that someone will take her grief seriously enough to prevent the next Zackery from climbing on top of a train.

“No one is stepping up,” she said. “I know he would want me to do that.”