Other favored lines include the A and 1, since longer lines offer more time to sleep between cleaning crews. “Each line has a culture,” said Muzzy Rosenblatt, the president and chief executive of the Bowery Residents’ Committee, a nonprofit organization contracted by the Metropolitan Transportation Authority and the city to send outreach workers through the subway system and major commuter rail stations.

“The mentally ill develop survival skills, and they know their best bet is the E,” he said. “We find more drug- and alcohol-addicted on the 1 because they are less conscious of the weather.”

During the recent cold spell — two weeks when the temperature didn’t rise above freezing — officials ramped up efforts to get people in shelters. The city’s Code Blue protocol was in effect, with the public encouraged to call 311 to get the homeless off the streets, and more than 100 outreach workers in orange parkas canvassing subway stations and transit hubs. They waited at the World Trade Center station, where they approached people during the roughly 15 minutes when the train is held in the station before it heads back uptown. “It gives us a small but significant window,” said Mr. Rosenblatt.

Workers do not approach people while the train doors are closed, he said, because they do not want them to feel trapped. “Many are obviously homeless and other people are less so, but these are fragile people,” Mr. Rosenblatt said. “They’re fearful, they’re distrusting. They’re struggling with mental health issues.”

He added: “It’s a different world at that hour. People erect structures in there.”