Downtown Chicago, the bustling birthplace of modern American architecture, a crucial crucible of the union between the speculative real estate deal and the aesthetics of the tall building, has in the past three years experienced its most spectacular building boom since the town redid itself in a new image following the great fire of 1871.

The city, ever assertive about its architecture, has got together a roadshow of large color photographs of new projects -- buildings recently completed, under construction and still in negotiation -- and put it on view in Washington in the glass-vaulted west hall of Union Station. The station was of course designed by Chicago's Daniel Burnham at his boisterous Beaux-Arts best.

This tribute from Chicago's architectural present to its transplanted past is a nice touch. And Burnham's restored masterpiece, a busy building these days, makes an ideal setting for so promotional a show. One gets an itch to take a ticket on the next available train in order to see the real things. The energy level is high in all respects.

For instance, there's a scale model (one of six in the show) of the Chicago Skyneedle, the city's latest entry in the world's-tallest-building sweepstakes. Though not actually built or even under construction -- it's ambiguously called a "project in progress," or PIP -- this is indeed a splendid splinter.

Designed by Cesar Pelli, the building is not the mile-high skyscraper Frank Lloyd Wright once proposed (presumably in sarcastic jest), but it's plenty tall at 125 stories, compared with 110 for the current record holder, the Sears Tower. The very idea may be nutty, but it's very Chicago in spirit: The Sears building, natch, is located there. Even as a model, Pelli's design of a tapering tower -- an efficient form that will cut down on wind loads -- makes an indelible, yearning impression. The architect calls it "a great poetic gesture on the skyline of Chicago," and the immodesty seems to fit.

Basically this is an exhibition of downtown office buildings, Chicago style, although a few suburban structures, institutional buildings and restorations are represented. It is good to learn, for example, that upon arriving by train one would find Chicago's own unique Union Station being restored and refitted. There's also a glittering glimpse of what one might expect deplaning at O'Hare Airport from a United jet -- Helmut Jahn's United Terminal, a fabulous sci-fi exercise in structural steel completed in 1988.

That the Chicago style is not what it used to be becomes readily apparent. That is, it is no longer a cohesive style based upon shared aesthetic assumptions. Rather, a vigorous variety predominates. It's true that some of the sillier designs on display -- Ricardo Bofill's bumptious classic revival curtain wall is a good choice as a bad example -- might lead one to conclude that architects are willing to hang just about anything onto these steel frame structural systems. But really execrable exercises are mercifully few.

Be they home-grown or imported, most of the architects here are engaging one another in purposeful debate. Although the evidence is limited -- lacking floor plans, site plans, cross-sections and other helpful devices, this is mainly a show of facades -- one does begin to sense that in Chicago as elsewhere there is a healthy new urbanism in the air. Irrespective of style, the best of these projects would seem to demonstrate a sensitivity to the specific conditions of a given site, to the character of the surrounding streets as well as to that of the nearby skyline.

This responsiveness affects the work even of individual firms. Skidmore, Owings & Merrill, for instance, weighs in with five or six projects here, each decidedly different. With its thin, beautifully detailed curtain wall of green reflective glass, the Quaker Oats Tower, completed in 1988, harks directly back to the SOM-led heyday of the International Style in the early 1950s. By contrast, the more recent NBC Tower reflects an older modern vocabulary -- with its setbacks and striking verticality, the building recalls New York's Rockefeller Center as well as nearby Chicago landmarks of the 1920s and '30s. And with their slightly peaked copper roofs and relatively deep facades, the powerfully paired towers of the AT&T Corporate Center, under construction, call to mind the sophisticated eclecticism of Burnham's era.

The various Chicago projects of the New York firm of Kohn Pedersen Fox are likewise impressive for their sophistication and their differences. The twin 50-story towers of the Chicago Title and Trust Center, under construction, are at once emphatically elegant and sonorously subtle. The facades of granite, metal and glass are wonderfully soaring, and they terminate at the top with a fretwork that'll seem downright delicate when viewed from the street. No pallid pastiche, this again is strong modern architecture with an engaging old-time feel. The other KPF projects here seem similarly distinguished.

In this context Helmut Jahn, for all of his occasional forays into historical quotation, seems clearly a modern artist retaining a passionate belief in the form-follows-function aesthetic. His River North Tower -- another PIP -- is the most singular object in the show. With its sleek, curving corner and its turquoise-colored exposed steel beams and brackets -- humongous brackets -- it is a back-to-the-future apparition with surreal appeal.

Jahn, too, evinces a firm grasp of urban dynamics. His design for North Loop Block 37 -- two towers of differing heights, linked at the base -- is 20th-century abstraction of a high order (literally and figuratively), its structure and its function apparent in every right angle of its facades. But besides being great grids, these buildings may become lively urban connectors -- that shared base houses a multilevel retail arcade.

I should say that to find this out about Jahn's design I had to consult the pages of the Metropolitan Review, a Chicago-based periodical whose September-October 1989 issue was wholly devoted to new Chicago projects. The exhibition itself is minimally informative about such critical matters. Even so, like the city it represents, it's exciting -- not a substitute for going there, but the next best thing. It seems to have had the entire city of Chicago behind it. Curated by critic Christian Laine, organized by the Chicago Athenaeum, the City of Chicago, the office of the mayor and the Mid-America Committee, and supported by a dozen or so private companies, it will remain on view through July 20.