A 74-year-old Leesburg, Va., grandmother today took on the task of ridding Grand Central Station of hundreds of flea-infested felines. Not quite the Pied Piper, she came out empty-handed, but hopeful, after her first trip into the tunnels.

"The cats just aren't hungry enough," said Anna Briggs, the founder and director of the National Humane Education Society in Leesburg. "We'll have to wait. It takes a trap, food, and patience -- lots of patience -- to catch a cat."

Patience prevailed when, on a second trip to check traps later in the afternoon, Briggs emerged with two cats and one kitten.

Early this morning, Briggs, a small woman with gray hair and an endearing smile, entered the dark, dank tunnels under Manhattan's Grand Central terminal, armed with cat food as bait for the cagelike traps she would use. She was there at the request of Metro-North Railroad officials, who asked her to find as many cats as possible and lead them out of the terminal tunnels.

Cats in need of medical care will be taken to the American Society for the Prevention of Cruelty to Animals, Briggs said. All others will go to the society's farm in Walton, N.Y., for sanctuary.

Metro North officials recently turned to Briggs for help with the cat problem that has plagued them since the 1960s. Fed by commuter and employe cat-lovers, the cats -- originally brought into the terminal to help control rats -- have reproduced at alarming rates.

Peter Stangl, the railroad's president, estimates that two to three hundred cats now live in the tunnels under the terminal. "We have rats, cats and fleas," said Stangl, who explained that the latest efforts to get rid of the cats began when more than 20 employes were treated for flea bites.

Dressed in blue slacks, a short-sleeved blouse and sturdy brown leather shoes, Briggs emerged after her first check of the nine traps she had set with the aid of her 25-year-old grandson, James Taylor, also of Leesburg, and explained that railroad workers and other cat-lovers probably fed the cats Friday evening. "You don't go into a restaurant unless you're hungry," she said.

No reason to fear, though. "If they're here, they will go in," she said assuredly, as she stood before television cameras and microphones.

An eviction plan was announced in July by Metro-North Railroad and traps were set, although railroad workers volunteered to wear special clothing or flea collars in order to enable the cats to stay. Only a dozen cats were caught and the project triggered a heavy flow of protest mail.

"Our efforts have been spotty," admitted Fred Palmer, Metro-North's superintendant of administration, who accompanied Briggs into the tunnels. "We're not trappers, we're railroad people," he said, adding that the company was hopeful that Briggs could get rid of all of the cats eventually.

This is not the first time she has gathered strays from Grand Central. Her Walton, N.Y., animal shelter -- the Peace Plantation -- is home to many cats picked up on previous prowls. In June, she caught 21 cats in four hours and in 1972, she captured around 75 felines from beneath the station. This time, she says she will be satisfied with finding 20 to 30 cats, and she promises to return every two weeks until the problem disappears.

She does not charge a fee; all she asks for is room, board and Nine Lives cat food for bait. She uses what she calls "a humane trap" that closes when the cat is far enough inside to avoid pinching the animal's tail. Individual traps, she adds, are used so that each cat can receive proper medical care.

Asked her impression of the onslaught of media attention, Briggs, who is well known in the Washington area for taking in strays at her Leesburg residence, answered patiently. "I'm astonished. But if it helps the cats, I don't mind."