San Francisco has long had plans to extend the Bay Area’s Caltrain line into the heart of its Financial District. Now, recent designs show that the transit facility could be extremely (and uniquely) green — which makes it even better news that the project just landed a $171 million loan from the federal government under the banner of the Transportation Infrastructure and Innovation Act.

The new building, which got approval for construction almost two years ago, would replace the old Transbay Terminal located in the crosshairs of Mission, Howard and Beale Streets. Right now, it serves as a hub for long-distance buses, and those running regular routes to the North and East Bay regions. Caltrain would be a major addition to its services.

Slated to cost $4 billion, the revamped Transbay Transit Center would integrate several ambitious green concepts. Not only would its 5.4-acre roof double as a city park — covered in grass and other plant life to aid with urban carbon capture — it would also be home to city-suited wind turbines (consistent with a report released last year encouraging S.F. to make wind turbines part of its famous skyline). The current blueprints also call for geothermal heating and cooling systems and plumbing engineered to recycle used water. It’s ventilation structure also breathes in fresh air automatically.

The federal funding is being used for phase one of the project, which includes an upgrade to Bay Bridge ramps, construction of a bus storage building, and preliminary preparations for underground transportation. This first phase is expected to cost $1.19 billion alone. Phase two will be the 1.3-mile extension of Caltrain, which has yet to be approved for government funding. Ultimately, it will also link Bay Area Rapid Transit (BART), MUNI and even California’s proposed high-speed rail line.

The project has also applied for $400 million in stimulus funds, arguing that altogether, the construction and completed facility could generate thousands of jobs.

Designed by Pelli Clarke Pelli Architects, the Transbay Transit Center is expected to be finished in 2014. It could serve up to 45 million passengers every year.