There certainly is a certain folklore surrounding the so-called mole people living in the New York subway. The shared fantasy is appealingly deviant, subversive, and romantic. Each time we climb down a stairwell and enter a subway station we are, in a way, leaving society ourselves. The rules change underground and frankly, we like it.
Of course the reality of homelessness is nothing like our romanticized version of resourceful underground communities squatting in the tunnels, homesteading in the darkness. imagine the situation in which living in a pitch-black subway tunnel is the best option available. There are some people for which this circumstance is reality.
There definitely are homeless people living in those subway tunnels. Several books and films document their lives but they are usually surrounded by controversy.
Dark Days (2000) -- A certain questionable mythology surrounds this remarkable documentary about a group of homeless living in the tunnels. Supposedly 20 year-old Marc Singer, who'd never before run a camera, lived underground with his subjects, recruited them as crew, convinced local merchants to donate equipment and even sold his own bed to buy film. His original goal in making the film was to fund its denizens' move out of the tunnel. apparently the DVD includes a 45 minute making-of feature and updates about the subjects' lives.
The Mole People: Life in the Tunnels Beneath New York City by Jennifer Toth -- she spent a year in the 1990s befriending and researching the lives of New York's underground homeless. (Some dispute many of the claims she makes in this book, specifically claims she makes about the existence of certain tunnels and abandoned stations. Her subjects may have fed her misinformation.)
The Tunnel: The Underground Homeless of New York City by Margaret Morton combines photographs with first-person oral histories of her subjects.
Living at the Edge of the World by Tina S. is a memoir of a runaway who lived in the tunnels under Grand Central Station for four years.