The subway rumbled toward its final stop north of Seoul. Along the way, hordes disembarked, with the determined, brisk gait of those with somewhere to be.

Far from the city center, the thicket of high-rise buildings grew sparser, and the afternoon sun crept deeper into the train cars, riding on an elevated track at that point. By the end of the line, many who remained on board were noticeably older — nodding off, gazing out the window, stretching their shoulders.

Lee Jin-ho — dressed smartly in a straw hat, white Adidas and a crisp hanbok — had taken two subway lines for more than an hour from his home to the last stop, Soyosan, on a steamy August day. He ambled about a hundred yards beyond the station, rested briefly in the shade — and then got right back on the train heading south.

An 85-year-old retired interior designer, Mr. Lee is one of Seoul’s throngs of subway-riding seniors, who take advantage of the country’s longstanding policy of free fares for people older than 65 and spend their days riding the trains to the end of the line, or to nowhere in particular, and sometimes back again. On long summer days — with Seoul’s temperatures averaging highs of more than 87 degrees in August — the air-conditioning is robust, the people-watching is engrossing and the 200 miles of subway tracks in the city are almost limitless in their possibilities for urban wanderings.

“At home, I’d just be bored, lying around,” Mr. Lee said.

Older adults who ride free of charge make up about 15 percent of Seoul’s annual ridership, according to data from the subway’s two main operators. The riders have become such an established part of the city’s fabric that they have a nickname — “Jigong Geosa,” derived from the phrase “free subway” — and the lines and stations frequented by them are well known.

Mr. Lee and his wife live in a cramped apartment, subsisting on a pension of a few hundred dollars, and his wife is largely homebound after five knee surgeries. For him, Mr. Lee said, there’s no better way to while away the days than to go for a free ride. The day before, he rode the trains in a loop — south to the end of Line 4, northwest to the last stop on the Suin-Bundang Line, back east on Line 1 — without setting foot outside the subway system.

“One spin around is exactly four hours,” he said.

He sets out on his own several times a week, heading for one of two stops equidistant from his home: Suyu Station is 1,100 steps to the north; Mia Station is 1,250 steps to the south. (He has counted.)

Riders like Mr. Lee say they know to abide by the careful rhythms and unspoken rules of subway riding: Avoid rush hour, when trains are packed and everyone is in a hurry. Don’t stand in front of seated young people, lest they feel pressured to give up their spot.

“You read, and doze off,” said Jeon Jong-duek, 85, a retired math professor who was riding with a volume on the theory of Chinese poetry tucked in his satchel. “It’s remarkably cool. There isn’t a corner of Seoul I don’t go to.”

Park Jae-hong, 73, who still works sporadically as a construction inspector and has been dabbling in modeling, said he found the subways meditative and relaxing. “For me, it’s an oasis,” he said.

There are six seats reserved for older passengers on either end of each train car, but Seoul in general seems to have less of a place for older people, even as South Korea is aging rapidly.

Cha Heung-bong, now 80, a former minister of health and welfare who proposed the free-fare policy around 1980, said many older South Koreans live on limited incomes because the national pension system wasn’t instituted until the late 1980s. About four in 10 South Koreans over 65 live in poverty, double the rate in Japan or the United States, according to the Organization for Economic Cooperation and Development.

Mr. Lee stopped working in interior design two decades ago when he couldn’t learn his way around a computer. He then took a job as a night guard at a middle school, where he worked for eight years — until the school told him he was too old for that, he said.

“You can’t keep up with the young folks,” he said.

With the subway system in a yearslong deficit, politicians routinely broach scrapping the free fares or raising the age to qualify. The mayor of Seoul, Oh Se-hoon, noted at a panel in February that less than 4 percent of the city was over 65 when the policy was adopted decades ago; now that age group makes up more than 17 percent.

“Do old people become old because they want to?” Kim Ho-il, president of the Korea Senior Citizens Association and a retired lawmaker, said at the forum. “We were nudged along into old age by the passing years.”

“Why are you trying to take this happiness away?” he asked, arguing that the country was saving more in health care by keeping older adults active.

On a recent afternoon, on shaded benches outside Soyosan Station, a rotating cast of older men who had each ridden the trains there alone sat around gabbing, their conversation meandering from history, to the economy, to South Korea’s stature in the world. Cicadas wailed discordantly, and trains periodically rumbled in.

“My apartment is so hot. On a day like this, the subway is a place of rest, a summer resort,” said Father Kim, a silver-haired 80-year-old Catholic priest, who wore his collar and black clerical garb with the sleeves rolled up as the temperature neared 90 degrees. He didn’t want to give his full name because, he said, he serves in Jesus’ name.

Han Kwei, 80, said he liked riding the trains in the early morning, when hard-working people were returning home after the overnight shift.

The men spoke of the leaner times of their youth. Mr. Han recounted working as a miner in Germany decades ago, as many poor South Koreans did at the time, and another man chimed in about his impoverished childhood, when few managed to eat three meals a day.

The seemingly trifling savings of 1,500 won per ride, about $1.15, is significant for their generation, and most would ride the subways far less if it weren’t free, they said.

Bae Gi-man, 91, said that after his wife of seven decades passed away last year, he spent days at home barely washing or eating. Subway outings motivate him to get dressed — he was wearing a polo shirt, slacks and a flat cap — and he eats and sleeps better after a jaunt, he said.

At home, he keeps five copies of the Greater Seoul subway map that he consults to chart his journeys.

“If I had to pay the fare there and back, I couldn’t do it,” he said.

By about 4 p.m. on the day he was riding, Mr. Lee was well on his way home. Glancing around the subway car that looked to be at least half occupied by older people, he said he agreed that the age for free fares should probably be raised.

“Seventy, 75-year-olds are spring chickens,” he said. “Sixty-five-year-olds are basically children.”