When the wrecking ball came down on San Francisco’s 71-year-old Transbay Terminal bus station in December, it marked the end of an era—and the beginning of a new one. The drab concrete structure will be replaced with the long-planned Transbay Transit Center, perhaps the most ambitious transportation hub to be built in the United States in the past few decades.

The $4.2 billion project, designed by Pelli Clarke Pelli Architects, includes a multimodal transportation hub, a 5.4-acre rooftop park, and a 1,000-foot-tall tower that will displace the Transamerica Pyramid as the city’s tallest building.

Touted as the Grand Central Terminal of the West, the transit center, scheduled for completion in 2017, will be the focal point of a new mixed-use downtown neighborhood south of Market Street. Construction is scheduled to begin in May.

Funding will come from a variety of sources, including sales-tax revenues, bridge tolls, and federal loans and grants. In January, U.S. Transportation Secretary Ray LaHood announced the project would get $400 million in federal stimulus funds, part of a $2.25 billion grant for a high-speed-rail link between Anaheim and San Francisco.

In 2007, Pelli Clarke Pelli won a competition to design the new transit center, which is being developed by the Transbay Joint Powers Authority. The glass-and-steel structure will occupy nearly five city blocks and accommodate buses, commuter trains, and California’s planned high-speed rail line, making it the nation’s first high-speed-rail station.

Planners say the terminal eventually will serve more than 45 million passengers a year. Passengers will walk through a light-filled, multilevel facility with undulating glass panels supported by a network of steel columns. The key design element, however, is the rooftop park. “That was probably the boldest part of our proposal,” says Fred Clarke, senior principal of Pelli Clarke Pelli. “And I think it’s one of the reasons we were chosen for the project.”

Designed by Peter Walker and Partners Landscape Architecture, Berkeley, California, the park will be open to the pubic and occupy the transit center’s entire roof. It will include walking paths, playgrounds, cafes, an amphitheater, gardens, and a 1,000-foot-long fountain with water jets triggered by the movement of the buses below. Visitors will enter the park via stairs, escalators, elevators, and perhaps even by funicular.

The park, Clarke says, will help make the transit center a “great neighbor,” not merely a functional transportation hub for daily commuters. “The history of transit centers in the United States is not a happy one,” he says. “They are seldom neighborly buildings—the Port Authority in New York is a perfect example. [Transit centers] tend to be blights on neighborhoods rather than generators.”

But John King, the San Francisco Chronicle’s urban design critic, wonders how many people will actually use the elevated park. “There’s a lot of skepticism about it,” he says. “It may be too large and divorced from the city to really work.”

Though still in the planning-and-review stage, the obelisk-like Transbay Tower will be directly adjacent to the terminal, allowing office workers to have direct access to the rooftop park. King calls the tower’s current design “a little generic-looking,” but Clarke promises an iconic building that will become one of the defining images of San Francisco’s skyline. Unlike the transit center, which has funding fully in place, Clarke says the tower is “at least one real estate cycle away.”

King says few will miss the old Transbay Terminal building, which had fallen into disrepair and decay. Designed by noted San Francisco architect Timothy Pflueger and completed in 1939 as the terminal for commuter trains that once crossed the Bay Bridge, the concrete structure had long outlived its usefulness, even as a bus station. “It wasn’t Grand Central Terminal,” King says. “There was no real push to preserve it.”