THEY call it the commuters’ secret, these denizens of the Terence Cardinal Cooke-Cathedral branch of the New York Public Library. It is located down a flight of stairs, just outside the turnstile entrance to the No. 6 train on the northwest corner of Lexington Avenue and 50th Street. The door is next to a MetroCard machine. There is no street-level sign announcing its existence.

“If you don’t take the train, you’d probably never even know this place exists,” said Eric Velasquez, 47, who commutes from the Parkchester section of the Bronx to an administrative job at a Midtown bank and stops by frequently. But the location is also a plus. “It’s second nature to return the book,” Mr. Velasquez said, “because you can’t help but pass the library every morning and evening when you’re getting the train.”

Then there are the people who assume the library is an outpost of the Metropolitan Transportation Authority. “They come in asking for help with the MetroCard machine,” said Anisha Huffman, the branch manager. “We do help them if we’re not too busy, and they also ask us for subway maps, so we keep a lot of them on hand.”

Before the branch opened in 1992, the space housed a library, dating to 1887, for the Archdiocese of New York.