Winter Storm Stella turned the New York City subway into a system of virtual sleeper cars for the homeless.

With many New Yorkers ​staying home from work during Stella, ​on​ some trains, vagrants even outnumbered straphangers.

“It’s concerning,” said Ron Caldwell, who was riding an A train at West 4th Street. “No one should be sleeping on the subway. For whatever reason, a lot of them have mental issues, so get them into a shelter.”

Many homeless people brought along suitcases and large bags, making it hard for some straphangers to find a spot to sit​ and leaving cars reeking​.

Huma Yara, 25, had a few pointed words for Mayor Bill de Blasio: “Instead of going around giving speeches, hire more people to take action,” she said. “When it’s this cold, you see it more, but it’s a problem. Take it seriously.”​

In the basement of ​the ​Rockefeller Center​ Sixth Avenue subway line​, a makeshift shantytown popped up inside one subway entrance, with as many as eight homeless people hunkered down for the storm near some MetroCard machines.

“This is really disgusting,” said commuter Menachem Fried, 24, of Flatbush. “You walk through here, you feel like you’re in a Third World country, not the heart of Manhattan. Like, how is it that this is the best option for these people?”

De Blasio said at a Tuesday press conference that homeless outreach workers were coordinating with the NYPD, FDNY and EMS to locate “anyone who may need to come in — either voluntarily or involuntarily.”

“If someone’s in the subway, they may not be in danger … but we still want to get them out of the subway and to some appropriate kind of shelter,” he said. “So that outreach effort will continue.”