150 bicycling "insomniacs," clearly a bit peculiar but probably not dangerously, came to town at 2:30 yesterday morning to see what Washington looks like under a full moon.

They arrived on the night train from New York, their bicycles stowed away in the baggage cars. When they emerged enmasse from the back of Union Station, they were greeted by four bemused Park Policemen, a few startled taxi drivers, and - bike lights flickering in the darkness like a mass of fireflires - 150 more "insomniacs" who showed up to join them on their tour.

Michael George, a CBS television editor, was blithely conducting an architectural history of the Union Station facade. "We have two types of Ionic columns here," he said into his megaphone, as the bikers swarmed out into the night. "This is known as the scamozzi order of Ionic pilaster . . ."

The occasion for this seemingly unreasonable behavior was Washington's first "Insomniacs' Bicycle Tour," organized by a group of New Yorkers who call themselves by that name and who make a practice of exploring Eastern cities in the middle of the night.

Sometimes they do this on foot and sometimes they do it on bicycles. They do it in large numbers, usually with a police escort or two, and they conduct their tours with an enthusiasm that suggests there are few things as satisfying as wandering around deserted city streets until the sun comes up.

"There's no better time to ride," one hiker cried. "This is the only time during the day when you can bike without being choked by fumes," said another. "Its safer," offered a third. A fellow standing with them scratched his chin for a moment and then said, thoughfully, "We're very bizarre people."

With that, they were off - a glittering train of spokes and leg lights moving quietly through the shadows of 4th Street NE toward the Capitol. It was 3:00 in the morning. A cool dampness lay over Washington, the end-of-summer late night air, and when the Capitol loomed suddenly in the distance a New York woman sucked in her breath and smiled, "Isn't it beautiful," she whispered. "Isn't grand."

And that is really what the midnight tours are about - pilgrimages to places hidden by the soot-dimmed frenzy of a workday in the city. To places made lovely again by the gentle illumination and privacy of moonlight.

"Everything takes on a softer glow," reflected Mary Beth Seidenfield, a New York economist, hugging her knees and looking around her with satisfaction. "Even Trenton loks at fantastic at night."

The procession forged up through Northwest, massing now and again for architectural history: Ford's Theatre, St. Matthew's Church, the District Building. Pimps and cabbies and delivery men, the early morning presence on dark city streets, watched from open windows.

"What's going on?" a cab driver demanded, poking his head out. Bikers, someone said, from New York. Lookimg at architecture and all. "Is that right?" the cab said, uneasily. He was trying to be diplomatic. "Well, it's not a bad idea," he said. "It's something to do."

It all began with Friends of the Parks, an organization dedicated to the appreciation of such fine things as Dutch elms and lovingly tended tulips. Friends of the Park has raised about $20,000 for tree planting and care in Central park in New York and four other parks since it was founded 12 years ago. The friends began conducting walks through the city and parks they loved, and one day, Robert Makla, a New York attorney who helped found the group, decided to try a new approach.

"These cities were all built before the automobile," said Makia, whose fervor for the classical beauty of marble buildings and sculpted parks was obvious in his tour commentary yesterday. "It's impossible to see the grandeur of these cities with all those automobiles whizzing by."

So - the night-time ramble. "This city is just incredible," Makla exulted, pedaling energetically along Massachusetts Avenue. "And the variety - Embassy Row - the Capitol was the most spectacular thing we've ever seen."

Across Dupont Circle, past three shadowy drummers and a small contingent of highly suspicious police ("Some guy stopped me and asked me," 'Say, officer, have you seen about 100 bicyclists?' I said, 'Go on, go home . . .'"). Past Lafayette Park, as the moon deepened to faded mustard and a husky glimmer swelled up in the east.

It's all so lovely, Makla kept saying through his megaphone - all of Washington is a park. If only the National Park would come to the Central park to make things right.

The bikers slowed a little, eyelids heavy. A New York lawyer named Paul Gareen remembered having toured Paterson, N.J. at midnight - "I always thought Paterson was just a street that you whiz through, but it has a waterfall," he said, sounding awed. "Ten stories high."

They said they might do Philadelphia next, and watch the neighborhoods wake up. They stretched and lay back, bicycles glinting in the muted first light of dawn. At the Jefferson Memorial, from his resting place between the Tidal Basin and the cherry trees, someone finally saw the sun.