A 38-year-old Brooklyn man was fatally stabbed early Friday after he and another man began fighting on a downtown No. 5 train in Manhattan during the morning rush hour, officials said.

The men began to argue after one stepped on the other’s shoe, according to a law enforcement official.

As the train headed toward the Brooklyn Bridge-City Hall station, the assailant stabbed the man, the law enforcement official said. When the train stopped at the station, the men got off and the attacker stabbed the victim a second time, the official said.

Officers responding to a 911 call about a stabbing at the station arrived at around 8:30 a.m. to find the victim, John Sheldon, unconscious on the platform with several chest wounds, the police said. Emergency medical workers took Mr. Sheldon to Bellevue Hospital, where he was pronounced dead. (The Police Department initially identified the victim as Sheldon John.)

The assailant, believed to be in his 30s, fled from the station, the police said. The investigation was continuing, and no arrests had been made as of Friday afternoon, the police said.

The police said they did not believe the two men knew each other.

At 10:15 a.m., a splatter of what appeared to be blood was visible inches from the edge of the station’s downtown platform. Officers milled about the blocked-off platform, as downtown trains, diverted from the station amid the investigation, rumbled past. Riders on the uptown side looked on, stunned and confused.

The killing, the first on the subway this year, came at a fraught time for the Metropolitan Transportation Authority, the state agency that operates the subway and is in a standoff with the Trump administration over funding for the transit system.

Sean Duffy, the transportation secretary, has waged a public campaign since February deriding the subway system as lawless and dangerous, and threatening to withhold federal money unless the M.T.A. addressed crime in the system.

But crime on the subway has declined, a trend that Jessica Tisch, the city’s police commissioner, has attributed to the deployment of additional officers on platforms and trains.

Major crime on the subway fell 18 percent in the first three months of the year, according to Police Department data. For the first time in seven years, there were no murders in the transit system in the year’s first quarter.

As of last Sunday, overall crime in the system was down almost 11 percent compared with the same period last year, although it has begun to rise slightly in recent weeks.

Still, after a series of random and jarring attacks in the system over the past two years, transit officials have struggled to dispel a persistent belief among some riders that the subway is unsafe. The killing on Friday was likely to add to that perception.

Some people have criticized the number of officers now patrolling the system, but Elizabeth Daley, who was waiting for a train on the uptown platform Friday morning, said she would like to see more.

“A lot of the time I would rather take a 30- or 40-minute walk just because once you’re in there and it’s moving, you can’t really get out,” Ms. Daley, 20, said.

Others shrugged off the episode, describing it as unsettling but not unusual.

“Obviously it freaks me out, but it’s New York. It’s a normal thing,” said Jonathan Ricket, a 19-year-old Pace University student who lives in Newburgh, N.Y., and commutes into the city daily for classes.

He paused for a moment.

“I’m just glad I didn’t see it,” he added.

This railroad track hasn’t carried a train across central Queens in 63 years, and it is more strange and more beautiful for it. The Long Island Rail Road’s Rockaway Beach Branch once offered a 30-minute trip from Manhattan to New York City’s ocean beaches. Along the way it traversed three and a half miles of parkland valleys, earthen embankments and concrete viaducts from Forest Hills to Ozone Park.

The line was abandoned in 1962. And so nature pursued its messy designs. Forests grew. Signal towers fell. Coyotes colonized the dark bramble. In Rego Park, a section of track came unmoored from its fastening pins, and the rock ballast eroded. The track swayed free in the wind. A seedling fell between the stones. It became a red maple tree that grew and caught the rail, folding the steel I-beam into its bulbous trunk.

On a chilly day last winter, Jason Hofmann leaned down, framed the scene with his iPhone and took a picture.

“I like the way the branches move in the wind — it creates interesting geometry with the railroad tracks,” said Jason, 17, who lives nearby and occasionally walks the abandoned tracks. “It feels like nature taking over a war zone.”

Most of the old train line is managed by New York’s Department of Citywide Administrative Services. The middle section, a mile-long stretch through Forest Park, is open to the public. The rest lies behind razor wire, wobbly fences and hillsides of poison ivy. A few neighbors improvise ways to get inside, as do a handful of people who sleep under tarps.

“It’s gorgeous,” said Travis Terry, who lives in Forest Hills, three blocks from the old line. “It’s been untouched for 60 years, so you have these great trees. You’re in a forest and then you think to yourself, ‘Wait, I’m in New York City!’”

The locals are joined by especially ardent urban explorers, some of whom take multiple buses and subway trains to get there. They enjoy the abandoned line for its decayed beauty, and because so few people know it’s there.

“It’s under the radar because it’s in Queens, and it’s hard to get to,” said Jeff Seal, a train-loving performer who filmed himself walking the entire line. The video has received 12,000 views since he posted it on YouTube six years ago, which hasn’t done much to raise the Rockaway branch’s profile. “I like that it’s hidden in plain sight,” he said.

The place may not remain hidden for long. Mr. Terry leads Friends of the QueensWay, a nonprofit that hopes to turn the abandoned train line into a linear park similar to the High Line in Manhattan. The group has received $154 million in grants from the city and the federal government, enough to complete the first mile and a half of park construction.

“It’s important for us to utilize every inch,” Mr. Terry said.

Because the Rockaway branch is in New York City, even this forgotten wasteland has become contested ground. A competing group, called QueensLink, hopes to restore train service aboveground, with a new tunnel connecting the branch to the subway system. Cost estimates start around $4 billion and balloon to $9 billion. The group has won $400,000 to study the rail idea from the U.S. Department of Transportation — the same agency that also gave $117 million to QueensWay to build its park.

“It’s a remarkable resource that has to be used,” said Neil C. Giannelli, 70, who has lived along the line for 24 years and supports QueensLink.

In the meantime, the Rockaway branch grows more beautiful for its disuse. To spend a year trespassing the old tracks is to enjoy the infinite overlapping riot of things planted and dead, built up and falling apart.

If the Rockaway Beach Branch becomes a park in Queens it will be thanks in part to the success of the High Line, another once-abandoned train line that opened as a park starting in 2009 and now attracts tourists from around the world.

Robert Hammond is a co-founder of Friends of the High Line, which led the redevelopment effort. To sell his vision, Mr. Hammond asked the photographer Joel Sternfeld if he might take a few snapshots of the abandoned train line.

Between his photo books and museum exhibitions, including a show at the Museum of Modern Art in 1984, Mr. Sternfeld was among the most celebrated art photographers in the world. He agreed to help the High Line, but not for money. His only request was time.

“I need a year of exclusive access,” Mr. Sternfeld recalled telling the group.

Mr. Sternfeld got his year, and keys to the abandoned line. The wet spring dried. Summer weeds bloomed. Though the High Line in the 1990s was a refuge for artists and teenage thrill seekers, Mr. Sternfeld’s pictures included no people. Instead they focused on faint trails bushwhacked into thickets of invasive Ailanthus trees. His pictures, full of leaden skies and muted auburn bramble, lent the High Line the mystique it needed to land powerful backers, Mr. Hammond said, including Diane von Furstenberg and Mayor Michael Bloomberg.

“For me, it was the seasons,” Mr. Sternfeld said in a recent interview. “I have always been interested in the seasons, and the changing of the seasons.”

A quarter century later, the project to redevelop the Rockaway branch is in the same early phase as the High Line was when Mr. Sternfeld walked it. Yellow lichens dot the rusted steel like polka dots on a necktie. Flocks of blue jays march down the canopy like columns of soldiers, squawking and unafraid. Behind an apartment tower in Rego Park, a high-banked ridge gave way and buried the rail line beneath a slow-moving avalanche of soil.

On a warm day last summer, Alex Cotter left the sidewalk by Yellowstone Boulevard and scrambled onto the steep embankment to the Rockaway branch, using tree roots as handholds to pull himself up. He hoped to see an opossum. Instead he was trapped, dense Ailanthus blocking all northern progress, greenbrier thorns to the south.

“I always thought it would be interesting to go up there, I just never actually did it,” said Mr. Cotter, 28, who grew up near the line in Rego Park. “Maybe I’ll come back when it’s cold.”

Beneath the Yellowstone Boulevard bridge lies a triangle-shaped lot that once was a dumping ground for televisions and car batteries. A decade of free labor turned it into the Compost Collective, where trash is sorted by volunteers and chickens peck one another in a double-decker coop.

At the collective’s winter picnic this December, Anuradha Hashemi stood in the shadow of the quiet train bridge and kept watch over her son’s first bonfire.

“Mom!” said the boy, Obi, 6. “My marshmallow is on fire!”

In the early 2000s, Patrick Mohamed bought a tall, narrow house in Woodhaven. His back patio ended in a cinder-block wall. Just beyond the wall, the Rockaway branch’s embankment rose in a hillside of weeds and trash.

Now Mr. Mohamed is 63, gray at the temples but still quick with his steps. A cold day in February found him in his driveway surrounded by steel barbells, completing his daily exercises. Mr. Mohamed walked to the back of his property, hopped two steps carved into the cinder blocks and climbed into his garden, on land appropriated from the old train line.

Raised tomato beds climbed the hill like a staircase. They were topped with trellises for long beans and bitter melons and small-gauge screen to keep the rabbits out. Where the hill crests, fat terra cotta planters filled with barren soil extended Mr. Mohamed’s domain all the way across the first set of tracks.

Everywhere else in New York, land rights are adjudicated to the square inch. On the Rockaway branch, things are looser. Some homeowners keep their backyards flush with the property line laid down by the railroad. Many have edged their fences back a few feet, claiming space for a shed, a foosball table or a rope swing. Few of Mr. Mohamed’s neighbors are eager to see the line repurposed as a park, which might bring nosy strangers to their backyard retreats.

“I’m worried about people looking in, hurting our privacy,” said Lasha Revia, 46, who carved a stone-lined terrace into the embankment where he hosts family gatherings in summer.

But no one else along the entire three-and-a-half-mile line has pursued a campaign of territorial expansion as successful as Mr. Mohamed’s.

“I built this over 24 years,” he said. “I did it a little piece at a time.”

One man can impose only so much order on a place so riotous. Rather than feel discouraged by the disorder, Mr. Mohamed greets it with delight. In summer, when the garden pruning is finished, he retires to his back deck. He watches darkness descend at its own celestial pace.

“At night it gets really dark back here,” he said. “We get great stars, and the moon comes out really clear.”

Much of the New York City region was under an air quality advisory on Thursday morning as smoke from one of New Jersey’s largest wildfires in two decades made its way north.

The fire, which has been burning in Ocean County since Tuesday morning, has grown to 13,250 acres, mostly in the heavily forested Pine Barrens. Earlier this week the fire forced officials to shut down the Garden State Parkway miles and prompted the temporary evacuation of thousands of people in Ocean and Lacey Townships, the state’s Forest Fire Service said.

Smoke from the fires spread over a large portion of New Jersey on Tuesday and Wednesday, prompting warnings about the air quality and at times irritating peoples’ eyes and making it difficult to breathe.

By Thursday, the smoke had spread to the north, prompting an air quality health advisory that was in effect through the end of the day for New York City, as well as Bronx, Kings, Queens, Richmond, Nassau, Suffolk, Westchester and Rockland Counties. The New York State Department of Environmental Conservation warned that the air quality index, a measure of pollutants, was likely to reach above 100, which means the air is unhealthy for sensitive groups like very young people or those with asthma or other respiratory problems.

By 5 a.m., the air quality index for the New York City region had reached 83, while Long Island was at 33. Newark was at 80 and Philadelphia at 77.

Southwesterly winds on Thursday afternoon were expected to spread the wildfire smoke toward Long Island.

“New York and Long Island, especially Long Island and the boroughs in New York are most at risk today,” said Brian Hurley, a meteorologist with the Weather Prediction Center. “Also southern parts of upstate New York, south of Albany and southern Poughkeepsie, but it’s really confined to the metro region.”

By Friday the smoke is expected to thin and clear, as stronger winds develop and rain spreads to the region later in the day.

There have been no injuries and no homes were damaged as the fire has spread west through the mostly forested area of the Pine Barrens, part of a containment strategy meant to protect homes closer to the coast. On Wednesday, the authorities said they expected the fire to grow even as efforts to contain it gained ground.

Fire authorities said the blaze may become one of the largest wildfires in the state since 2007, when a flare dropped by an F-16 fighter jet ignited a fire that ultimately consumed 17,000 acres. The cause of this fire is still under investigation.

Much of the state was at elevated risk for wildfires this week, and by Wednesday the entire state was under a “high” rating, the middle point of a five-point scale that the state uses. Fuel like dry grass that would help any fire spread more easily has been especially dry, as the southern part of the state remains in drought conditions.

Rain toward the end of the week is expected to give firefighters a helping hand in bringing the fire more under control.

“There’s going to be slight chance of rain later Friday and into Saturday,” Mr. Hurley said. “Saturday is going to be the day with the best chance of rain. So that will definitely help.”