The most enduring and useful custom of New York subway riders is that they don masks of stone at the turnstile, and keep them on until they’ve gotten where they are going. The origins of this sound practice are beyond the memory of any living New Yorker, but even if it began with Peter Minuit, its value continues to be proved every day.

And so.

On a Thursday night at the end of March, three people boarded the same car of an uptown No. 6 train at Spring Street. Eitan Noy, 25, a D.J. at heart and construction worker by day, had just come from an art gallery on Mulberry Street in NoLIta and was making his way home toward Sunnyside, Queens.

Charles Sonder, 24, an architect, had left a bar on the Lower East Side and was going to meet friends. A few convivial hours in the first place had given him an appetite. For sustenance on his train ride, he grabbed a stack of cheddar Pringles and a bag of Gummi-Bears. He wasn’t proud but made no apologies for his diet. “We’ve all been there,” he would later note.

Sitting next to Mr. Noy was a woman, age unknown, but a safe guess would be in early 20s. A fourth party would make a late entrance. “This dude, at the last moment before the doors closed, stepped in,” Mr. Noy said.