Pennsylvania Avenue, that stretch of it between the Capitol and the Treasury, is strangely beautiful in the early darkness of December evenings -- spooky but spectacularly alluring, dressed for a party that seems always on the verge.

Last Tuesday, coming out of the National Gallery with the intention of heading home on a subway, I found myself pausing to marvel once again at the swath of this street with its lighted monuments at either end, an accustomed sight but never more entrancing. Someone, doubtless at the behest of the Pennsylvania Avenue Development Corp., had taken the trouble to wrap the street light standards with spirals of plastic greens and little Tivoli lights, a poignant gesture that added a subtle sparkle to the already impressive array of lights, colors. Main Street U.S.A. and eerie grandeur. I decided to walk.

Predictably, it was a solitary trek, inspiring, depressing and odd. Winter evenings can be magic moments in cities. Buildings mysteriously reverse themselves in appearance, still-lighted windows becoming decorations suspended in the night. There's daytime busyness still in the air but an expectancy, too, of pleasurable appointments being kept. Any adult with a full meal in his stomach, or the assured prospect of getting one, can feel young.

Of course, this special slice of the nation's capital isn't quite a city, yet. There's immense promise in the great vista, excitement in the idea of a city in the remaking. But if one starts as I did near its eastern end one will encounter curious, even awesome, sights. What could be stranger than Arthur Ericson's Canadian Embassy building, its scaffolding recently removed, looming inner-lit in the night like a Piranesian dream, and combining, with dream logic, visions of Arcadia and 20th-century monumentality? An amazing, only-in-Washington apparition (and only for the next few months). Some buildings -- most buildings -- look better before they're done, and even better in odd, romantic, transforming light. One hopes, without fully trusting, that this will not be one of them.

And, while passing by bleak holes of construction and buildings that look convincingly bombed out, one must be quick to spot signs of life. Across the negative space that will become Market Square, for instance, there's a glimpse to catch of the Stephen Antonakos neon-mural on the ground floor of the old dime store where the Washington Project for the Arts used to be -- a reminder that someday, maybe, this will be an urban arts district worth the wait.

Definitively, this walk will be made alone and in a weird bucolic hush punctuated only by an occasional horn or a screeching tire or, perchance, the self-important clamor of fire engines. My sole companions on the sidewalk were a few shadowy souls aiming themselves determinedly for the Metro station at Market Square. More than 50,000 people work in this unneighborly neighborhood, but where do they go afterward? Where's the party? Not here.

In the 600 block, north side, where Barney's eatery used to be, there's a new building, a year-old building, with shipshape storefronts pleasantly lit and waiting for tenants. Next door, in storefronts painstakingly restored to an idea of their 19th-century origins, one of which was occupied by the funky Apex liquor store, handsome wooden desks are visible through fine curtains. Offices, not stores.

This is a north-side story that goes on. North side, because the south side of the avenue long ago was turned into the Federal Triangle office compound. Despite plans and plans and lots of talk, nothing yet has been done to mitigate the chilling effect this has had on evening activity in the downtown.

It's hard, especially at night, to walk the long walk in front of the FBI Building without noting for the umpteenth time the structure's profound anti-urbanity. Will those concrete panels ever be busted down, to be replaced by glass and, behind that, commerce? Not likely. More troubling are the still-empty storefronts at 1001 Pennsylvania, one of the handsomer new office buildings in the city, and the vacant restaurants in the sleek Heurich Building at 1201.

Thank heaven, at least, for the Pavilion at the Old Post Office Building across the street. Tourist business inside the magnificent shell seemed desultory on Tuesday -- one hopes it's the time of year. Even the clarinet player on his stage seemed understandably confused, almost apologetic, about being there. But locals had taken over the bars in Fitch, Fox & Brown's and Blossoms, where the lively scene recalled what used to happen at the Blue Mirror II (long gone, of course).

And thank goodness, too, for the Willard and J.W. Marriott hotels. The very western end of the avenue is lovely, these nights, and almost perky, although an air of uneasy artificiality inevitably attaches itself to such sights as an unhappy flunky doling out roasted chestnuts to a cluster of PR types outside the upscale stores next to the Willard. Could these stores, too, be faltering? My guess is, you bet.

It is indeed a worry, this continuing tale of commercial listlessness and vacancy on the premier street -- an entire segment of the city is being replaced and it's too early to tell quite with what. There's no way to subsidize friendly seediness -- the time of establishments such as Barney's is over, done, finished. But could it be possible that another, fancier, more creative and more spirited avenue is in the making?

Actually, yes, and not simply in the sense that anything is possible. One can stand at Market Square, in front of those silly-looking Navy Memorial flagpoles, and imagine the building that someday will rise behind them, and the space in front of this building, truly Washington in size and corny grandeur, filled with people intent on being seen, gawkers from the sticks, nay-saying New Yorkers -- all manner of person. One can do this because this amazing boulevard nearing its 200th year is really, really beautiful. Especially on mild Tuesday evenings in December.