What’s Worse Than Heat and Humidity? Heat and Humidity on the Subway.

You make a mad dash out of one subway car and into another before the doors slam shut. Or you use your hands as fans. Or you sit by an open window, hoping for even the slightest hint of a breeze. Or, resigned to your sweaty fate, you call out to fellow commuters, warning them not to step aboard: “It’s very hot!”

Such is the bleak experience of some rare but unlucky New Yorkers in this especially steamy season: sweltering inside a subway car whose air conditioning has conked out.

On a recent Wednesday, when outdoor temperatures reached above 90 degrees and felt even hotter with the humidity, Car 1859 on the No. 1 line was one of those cars.

It was not the first time.

The car, which is about 37 years old, has been taken out of service four times because of reported air-conditioning problems over the past six years, according to the Metropolitan Transportation Authority.

Yes, it is generally uncommon. After all, said Richard Davey, the president of the New York Transit Authority which operates the M.T.A., the air-conditioning on all of New York City’s nearly 6,000 subway cars works 99.4 percent of the time. That percentage is 8.5 percent better than last year, and 33 percent better than two years ago, he noted.

“For each of our customers, a tough experience is a tough experience, but a vast majority today, they’re going to be on a pretty comfortable car,” Mr. Davey said.

Still, that is cold comfort for those stuck in a hot car on a hot day.

Lulu Jenkins, 38, was perhaps among the unluckiest of riders on Wednesday. By 2:15 p.m., she said she had already been in three trains without air-conditioning.

“I’m trying to hurry up and get off,” said Ms. Jenkins, who is from Harlem, as she stood near a door inside Car 1859 as it headed north. “It’s really hard, and when there’s no A.C. and there’s a bunch of people coming in, it makes it even worse.” A nearby window was cracked, elevating the clang of the train on the track but doing little to dry the beads of sweat forming on her face.

Ms. Jenkins said she needed to ride the train just four stops downtown, but that if she needed to take the train to work and spend more time on it, she would switch to another car or take a different line altogether.

As time passed and the temperature rose, fewer and fewer people were sitting on the rapidly warming seats. As soon as new passengers felt the air inside, they jolted back and raced to a less oppressive car.

Well, not everyone. Wilfred Same, 57, said there was no reason to move. He said he often takes the No. 1 train to his job as a security guard and was on his way there on Wednesday.

“Many people are running away because it’s hot,” said Mr. Same, who is from Senegal. “It’s crazy.”

The air-conditioning on subway cars can stop working for various reasons, Mr. Davey said. The train can run out of Freon — a gas that is used in air-conditioners — or a filter can become clogged.

Sometimes the car is just old. Older cars are clustered on the A, C, No. 1 and Rockaway shuttle lines.

The M.T.A. is buying thousands of subway cars and is currently putting new ones on the A line. The new cars have automatic temperature sensors that notify officials if they are hot.

In the meantime, Mr. Davey encouraged riders to report hot cars to the authorities; repair time is between one and six hours.

“We do have roving managers and supervisors out with a temperature gun,” Mr. Davey said. Workers aim their monitors at thermometer stickers on the ceilings of train cars, testing them before they depart and while they are in service.

Kevin Blucher, 48, said he often encountered hot trains on the No. 1, 2 and 3 lines.

“It can be dangerous, especially the people who have health problems and all of that,” he said.

But for Mr. Blucher, the heat was manageable. “I’m from the Caribbean,” he said.

Rita Walters, 59, let out a sigh as she walked onto Car 1859 and then a giggle as the doors closed before she could exit. She said the train felt like a sauna.

“I come from a tropical people so I like hot weather, but this is a little much, even for me,” Ms. Walters said. She said she typically changes cars immediately. But because she was traveling just three stops, she said she would probably stay put. She estimated that she encountered a hot car once a month.

Riders began demanding air-conditioning on subway cars in the postwar years. Air-conditioning came to the transit system in 1955, and the city purchased 600 air-conditioned cars in 1967. But it was not until 1993 that the Transportation Authority was able to boast that 99 percent of its cars had air-conditioning.

While some subway cars are occasionally hot, many subway stations are often stifling. Tiffany-Ann Taylor, the vice president for transportation at the Regional Plan Association, said that most subway platforms, barring the new ones such as at Hudson Yards, are hot because the air-conditioning on the trains generates heat that is pushed onto the subway platforms and then trapped. She said that heat generated from the cars moving also makes the stations hot.

“Our stations weren’t really designed for something like being cooled, and so it’s perhaps not what folks want to hear, but it’s kind of the system that we have today,” Ms. Taylor said.

Back on the No. 1 line, Sydney Allard, 24, was waiting for an uptown train on Wednesday afternoon with a tennis racket in tow. She said hot cars were just a part of living in the city, though she had not encountered one this summer yet.

Still, she has a method of distracting herself, should the occasion arise: She tries to read or do a crossword puzzle until she is able to get out of the car and back into the open air.

“It’s very sticky,” she said, “ a little unpleasant.”

Ora Shtull, too, manages to take things in stride. She lives near the No. 1 train and just switches cars when she encounters broken air-conditioning.

“I’m just grateful for having access to public transportation,” she said.

Dodai Stewart contributed reporting.