Sporting a "Save Grand Central" button on the lapel of her white wool blazzer, former first lady Jacqueline Kennedy Onassis swept into town briefly yesterday on behalf of the legendary New York City station and, appropriately enough, by train.

Not just your everyday Amtrak from New York, mind you, but a seven-car special on which she and more than 300 fellow passengers who had paid $60 a head ($25 for children) traveled to Washington to call attention to a Supreme Court hearing today on the "landmark" status of the station. Penn Central Railroad, which owns Grand Central, would like to transform it into an office tower, but may not do so if it is officially deemed a landmark.

At a carefully orchestrated press conference in Union Station's main waiting room here, Onassis, wearing a navy blue dress and scarf, flat navy sandals and a navy-banded wristwatch in addition to the white jacket, told her audience, "I can't count the times I caught the train to Grand Central. It was never just another station."

In her whispery voice she breathed almost inaudibly that, "We all realize how important Grand Central Station is. Why should it be torn down to be replaced by parking lots?"

During the leisurely, four-hour trip from New York, passengers lunched on McDonald's hamburgers as a Mozart trio, two mimes, a pair of clowns from the Big Apple Circus School, fire-eating jugglers and a strolling banjo player entertained. Onassis made one stroll through the train, chatting briefly with the passengers supporting the effort of the Municipal Arts Society of New York to preserve the station.

She was met in Washington by Joan Mondale and New York Sen. Daniel Patrick Moynihan, who participated in the press conference along with author Brendan Gill, fellow passenger and a leading member of the Save Grand Central committee.

Following the press conference, the group moved slowly, en masse, to another room for a reception. Onassis, however, stayed long enough only to pose for the TV cameras before starting for the door. On her way out, she was stopped by a tall, thin young man with a gold earring in his left ear. "Oh, how are you?" she greeted Bill Paley Jr., son of CBS chairman Bill Paley.

"She's an old friend of my mother's," explained Paley, who said he was there because his Grandy Dancer restaurant had donated the juice, coffee and cake for the reception.

Onassis then swept into a waiting taxi - to catch a plane back to New York.