Salesforce Transit Center consolidates operations of 11 bus systems in the Bay Area and to serve as a future rail terminus for the Caltrain commuter rail and California High Speed Rail connecting San Francisco to Los Angeles. Anticipated to serve over 45 million people annually, the transit center has become a transportation hub and catalyst for a new, vibrant Financial District in a once underdeveloped neighborhood in the city. The 1.5 million-square-foot facility is a large-scale piece of infrastructure designed to accommodate multiple transportation modes. The project brief called for a long horizonal facility spanning four city blocks that would spur vertical development. The design team immediately realized that adjacent towers would focus attention squarely on the roof of the transit facility. Asking how the roof become more than a mechanical penthouse, led the design team to propose a green roof that soon became a park. The approach transformed the facility from a terminal for arrival and departure into a community resource that would become a destination. The new 5.4-acre rooftop park, now known as Salesforce Park, extends a hallmark of San Francisco’s historic neighborhoods, which grew up around public parks. Accessible from eleven entry points, the park provides public green space to the tenants of adjacent buildings and the citizens and visitors of the city. Through extensive programming that includes a children’s playground, cafes, an 800-seat outdoor theater, public art installations, and extensive local vegetation that frames the 1.5 walking trail on the parameter of the park, Salesforce Park is a hub of activity in the heart of the new Transbay District. A multi-modal transit center is a concentrated source of exhaust from cars, buses, and trains, and due to its 24-houra- day activity, it uses resources and generates pollution at a rate much greater than other buildings. The rooftop park became an opportunity for carbon capture and natural air filtering through broad leafed vegetation. Instead of a massive roof absorbing and radiating heat, the 5.4-acre rooftop park absorbs carbon dioxide from bus exhaust, filters stormwater, and provides a habitat for local wildlife. With more than 600 trees and 16,000 plants, the ecosystem captures 12 tons of carbon annually. The Salesforce Transit Center is built to LEED GOLD. Environmental sustainable approaches that were implemented include extensive use of natural light that flows from the park down to the train box to reduce energy costs, greywater and stormwater reuse that saves 12 million gallons of potable water annually, natural ventilation, and a geothermal system with ten miles of piping that saves 350,000 gallons of water per year (60% less than a typical cooling tower system). Salesforce Transit Center recognizes that connectivity and sustainability are essential building blocks for increased economic development and cultural vibrancy in twenty-first century cities. Phase Two of the project will see a 1.3-mile Downtown Rail Extension that will bring Caltrain commuter rail from its current terminus at Fourth and King Streets and, ultimately, welcome California’s High-Speed Rail.