One of Mayor Bloomberg's biggest development initiatives has been the Hudson Yards project, which turns space over the West Side rail yards into commercial and residential buildings. Developer Related Properties is working the parcel west of 10th Avenue while Brookfield Properties is tackling a lot between 9th and Dyer Avenues and West 31st and 33rd Streets. Brookfield, which calls the project, Manhattan West, broke ground on its $680 million platform above the rail yards—and the building will occur while trains are still moving in and out of Penn Station. Check out this video showing the deck engineering:

Here's how Brookfield Properties explains the project, "The platform, a series of 16 bridges, completes the surface upon which the entirety of the 5-million-square-foot development will rise, envisioned as two two-million-square-foot Class A office towers, a residential tower, a 1.5 acre open public space, and retail throughout. The cores of the towers will be constructed on bedrock to the north and south of the platform. The total site comprises five acres, of which the platform will occupy 60%. The Manhattan West deck is being constructed utilizing post-tension pre-cast segmental bridge technology which minimizes disturbance to train operations below."

The platform project alone will generate more than 600 constructions jobs ("$52.7 million in employee compensation and $4.7 million in tax revenue for New York City, New York State").