With no prior training in film, 21-year-old Londoner Marc Singer set out to make this 16-millimeter black-and-white documentary about the homeless people living in the tunnels under New York’s Penn Station. Singer’s six-year quest–including a brief stint of being homeless himself–deserves notice, and in a way I’m disappointed that the film omits it. But what’s most remarkable and fascinating here are the squatters, who do a pretty good job of explaining themselves without any outside narrator (and who, in countless ways, assisted Singer in shooting the film). The lives of these people inside their shacks are full of surprises (one keeps several dogs as pets, another shaves with an electric razor and a broken mirror) as well as grim confirmations (the self-loathing misery of a crackhead who lost her children in a fire), but the things we don’t know about them also significantly shape our experience of the film. Their underground sojourn came to an end when Amtrak evicted them and the Coalition for the Homeless found them normal housing. Despite its title, the film seems excessively (or at the very least prematurely) cheerful in its closing stretch. Still, this is an eye-opening tale of how part of our population lives, and as an authentic image of material suffering it makes something like Lars von Trier’s Dancer in the Dark seem even more dubious. 84 min. Music Box, Friday through Thursday, October 20 through 26.