Nine years have passed since Fred Clarke and Cesar Pelli conceived the design notion of a San Francisco transit center wrapped in an undulating skin and topped by a quarter-mile-long park.

Now the two leaders of Pelli Clarke Pelli Architects can stand inside and look out through sections of that skin. The glossy visions of 2007 are becoming reality and, so far, the pair’s architectural ambition has shaken off the politics and budgetary problems that still could pull it down.

“It’s going to really glow,” Clark said a few days ago as he stood on the elevated open-air deck where buses someday will arrive from the Bay Bridge, a space screened by aluminum panels punched through with a crystalline pattern and coated in iridescent white paint. “Look at the sparkle on the edge of the punches. You can design a space and simulate it (in computer renderings), but you don’t have any idea that will happen.”

The screen’s not the only hint of what will greet us by the end of 2017, when the huge complex that runs behind Mission Street from Beale Street west nearly to Second Street is scheduled to open. The structural braces along its outer edge are being painted a thick, clean white. Inside their crisscrossed grid, glass storefronts for future retail shops are being installed.

This smooth progress is a counterpoint to the spring’s flurry of stories about the transit center and its daunting price tag.

Pleasant commute touches

The budget for the first phase is now $2.4 billion, twice the amount estimated when the competition was held in 2007, driven higher by a local building boom where construction firms can name their price for unusual projects. In April the longtime director of the Transbay Joint Powers Authority, Maria Ayerdi-Kaplan, was sent packing so that City Hall could take a more direct role in keeping costs and timelines on track.

That’s the news. The reality is the efficient way the vast structure has emerged from the ground, taken form, jumped streets. Better yet, the finishes beginning to be applied — the layers that visitors will encounter long after the politics fade — offer the promise of a lyricism to the machine-like mass.

For evidence, walk south on First Street from Mission and take a right on Natoma: The curves of perforated aluminum are a smooth-flowing horizontal river amid the verticality on all sides. In direct sun, the skin is a bright, shimmering canvas. When the sun moves behind the panels, light streams through to give the screen and its spidery bracing the look of a taut sculpture.

Taut — but lithe, unlike the heaviness of the perforated steel panels that cloak the San Francisco Federal Building at Seventh and Mission streets. That slab-like tower, designed by the Los Angeles firm Morphosis, can be exhilarating. It also can turn brooding and grim.

Emphasizing white, not gray

Given how the transit center extends over streets between alleyways, the sinuous white wave makes sense.

“We designed the frame with triangular pieces because it’s the simplest, lightest way” to support the aluminum skin, said Clarke, citing Buckminster Fuller as an inspiration. Similar care went into the structural bracing’s soft hue: “We chose a slightly gray color so the white is what you focus on. The gray disappears.”

This clarity is a hallmark of the Connecticut firm founded by Pelli in 1977. So is pragmatism, as when Pelli and Clarke and the rest of the Transbay team changed the skin from glass to metal in 2013 in response to concerns about cost and security (think: mangled metal versus falling shards if a bomb is detonated). The change could have been bleak. Instead, it’s turning out to be an improvement.

“I think I actually prefer the (white) metal to glass — with a facade as important as this one is, making it so visually striking is a real plus,” said John Rahaim, the city’s planning director. He toured the complex-in-progress last week and, like me, was impressed by what he saw. “In general, my reactions were quite positive. The size and scale is really strong.”

Aside from Pelli Clarke Pelli’s attention to detail, the saving grace for the transit center is that the political clouds didn’t gather until nearly all bidding for construction and materials was complete. If City Hall had taken the reins a year earlier, the quest to trim costs by cheapening the finishes could have grabbed a few daily headlines but dumbed down what we’ll be living with for decades to come.

Was there value-engineering along the way? You bet. It looks as though a line of aluminum panels was trimmed from the bottom and top of the facade, turning an evening gown into a cocktail dress.

Overall, though, the evolving edifice retains the visual power that’s needed in this constrained setting. The low-slung form amid towers is shaping up to be a centerpiece, which is essential — because the concept of a Transbay “district” is no longer merely the pipe dream of planners and policy wonks.

Month by month, the skyline and sidewalks around the transit center are in transition: The steel frames of two towers are rising along its rippled form, each including a bridge that will connect to the rooftop park. There’s a new apartment tower on Fremont Street one block to the south. Another on First Street has just started construction.

To be sure, design perils lie ahead.

The park will be installed next year, and if the design by PWP Landscape Architecture of Berkeley isn’t as lavish as the initial vision — welcome to the world of architectural competitions — it offers a tapestry of landscapes and habitats. That acclaimed firm, however, no longer is designing the plaza at Mission and Fremont streets that marks the main entrance to the transit center.

Crucial role in daily life

The plaza is part of the Salesforce Tower project starting to scrape the sky next door, and the development team of Boston Properties and Hines apparently has let PWPLA go. The idea is to get rid of the redwoods in the approved design and open things up.

“We’ll be keeping close tabs on that,” Rahaim said. Good thing. That plaza needs to function in part as the ground-level foyer to the park set 70 feet in the air. If a new design turns the open corner into what feels like the quad of a Salesforce campus, with nothing about it that invites people to explore what’s up above, the potential of the publicly funded park above the transit center could be lost.

The role of that landscaped rooftop isn’t to look good from nearby towers, but to be a vital part of the everyday city. The closer we get to opening day, it’s easy to lose attention of the final round of decision making. The thing is, this can be the most important round of all.

John King is the San Francisco Chronicle’s urban design critic. Email: jking@sfchronicle.com Twitter: johnkingsfchron