Two teenage girls were discovered dead on top of an incoming J train at a Brooklyn subway station early Saturday and appeared to have been subway surfing, the deadly, social-media-fueled trend popular among some New York City youth, the authorities said.

Officers responding to a 911 call found the girls unconscious shortly after 3 a.m. on the roof of the last car of a train at the Marcy Avenue station in Williamsburg that had just crossed the Williamsburg Bridge from Manhattan. They were pronounced dead at the scene. Their names and ages have not been released.

“It’s heartbreaking that two young girls are gone because they somehow thought riding outside a subway train was an acceptable game,” Demetrius Crichlow, the president of New York City Transit, said in a statement on Saturday.

Subway surfing, in which people ride atop or hang off the sides of fast-moving trains, has been around since the transit system’s earliest days more than a century ago. But it has grown more deadly in recent years, especially among teenagers inspired by sensational videos of the practice, city officials say.

Through August, three people died this year while trying the activity, according to police data, including a 15-year-old boy found on the roof of a 7 train at Queensboro Plaza in July. Six train surfers died last year, up from five in 2023. By contrast, during the five-year period from 2018 to 2022, only five people were killed in suspected subway surfing incidents.

City and state officials and the Metropolitan Transportation Authority have ramped up efforts to dissuade young people from subway surfing. Since 2023, the police have used drones to catch subway surfers in the act. The M.T.A. has been working with social media platforms, including Instagram and TikTok, to remove subway surfing footage.

On Saturday morning, hours after the girls’ bodies were discovered, the Marcy Avenue station had returned to its usual busy cadence.

Duran Walker, 47, who heard about the episode at a laundromat below the elevated station, said he was heartbroken to learn that the victims were teenagers and said he doubted the accident would discourage others. “Even though two kids just passed away, they’re going to still do it,” said Mr. Walker, who has two teenage children of his own. “It don’t stop, unfortunately.”

Esrin Boran, who works at a fruit stand under the station, said that he did not understand the attraction of such a dangerous pastime.

“What do you think this is?” Mr. Boran, 38, said. “If you die, your mother is feeling the baddest in all her life. You’re dead.”

Olivia Bensimon contributed reporting.