NEW YORK — On a dark ledge under the Grand Central Station Terminal, just five feet directly above the IRT train's deadly electrified third rail, a fine stream of sunlight filters down through 20 feet of stagnant air onto a makeshift table. It illuminates a bouquet of flowers and a book by W. H. Auden.

The only other intrusion into the 9-by-9-foot compartment recessed in the tunnel wall is the thunderous quake of passing trains. A flashlight beam stops in a far corner of the room, where a few white plates rest on top of a working toaster-oven and a small refrigerator. An abandoned doll lies on the floor, which is carpeted with old clothes. The doll's dirt-smudged eyes peer out under shiny, well-stroked hair.

"They were just here," a guide says, smelling the last drops of milk in a bowl. "They're probably still here watching us."

New York's homeless fearfully speak of the residents of these and other dank underground havens as the "mole people."

Although some homeless people have found haven in holes burrowed under subway platforms and on catwalks above the roaring trains, the "mole people" are in a different category. Outcasts in a world of outcasts, they have gone deep into the city's nether world of interconnecting railroad, subway and utility passages, setting up homes and even communities that can number into the hundreds in population.

Estimates of the underground homeless population vary widely. One transit official speculated that up to 25,000 people live in New York's underground tunnels. A Transit Authority report to the City Council estimated that 5,000 people live in the New York subways alone. But Marsha Martin, a professor of sociology at New York University who headed the report, said that figure is a guess at best.

Transit authorities have one gruesome barometer: Trains derail when they hit a body. That now happens more than once a week, according to one transit official who asked not to be identified.

"That's often how we discover places where they live--when someone rolls onto the tracks (while sleeping) or touches the third rail" when walking along the tracks, said Lt. John R. Carlo of the New York Metropolitan Transit Police.

Cardboard shanty towns in the tunnels have also started fires along the tracks. Moreover, transit authorities say, some vagrants have started to threaten subway maintenance workers.