Hudson Yards

From this:

Long Island R.R. Yard on west side of Manhattan

To this:

Proposed Site Plan

Eric Jaffe:

The phrase "only in New York" is probably overused, but there are times when it still applies. A plan to build an entire 26-acre neighborhood with 17 million square feet of buildings atop two platforms suspended over an active rail yard serving America's busiest train station is one of those times.
The neighborhood will be known as Hudson Yards, and construction officially began today on the first of those platforms — over the eastern part of the rail yard. That platform will ultimately hold two office towers, two residential towers (one of which will have a hotel), a million square feet of retail, and about five acres of open public space. And it will all come together as 30 Long Island Railroad tracks remain in operation to serve commuters through Penn Station.....
The key to it all will be the platforms. Jim White, the engineer in charge of the platform construction, says 3D modeling helped identify places in the rail yard where caissons could be drilled all the way into the bedrock without disrupting the tracks. These 300 caissons, each installed with 90-ton cores encased in concrete, will serve as a foundation for load-bearing support columns. At the "throat" of the yard, where the 30 tracks converge into four to enter Penn Station, long-span bridge trusses will shoulder the weight.
All the work must be done in conjunction with Long Island Railroad to ensure the trains can continue their normal operation, says White. (The trains moving into and out of the yard don't have any passengers on them; it's effectively a parking lot.) Cross says the Hudson Yards development crew can close down any four of the main 30 tracks at a time for work but are limited to weekend-only closures of the throat, coordinated carefully with LIRR.
The time and space constraints demand an uncompromising efficiency on the part of the construction team. At some points, says White, workers will have only an hour or two to mobilize rigs weighing hundreds of tons into position, drill caissons into place, and remove all the equipment before the trains pass through. As if that weren't hard enough, the job requires an incredible degree of precision; White says the engineering tolerance is often just an eighth of an inch. (And you thought your Ikea desk was tough.)

I would not want to be the project manager on this job.  The article goes on to say that Park Avenue in the vicinity of Grand Central Station, is also built on a platform over the railroad tracks entering the station.  I did not know that.