How are large skyscrapers and buildings constructed on top of a railroad yard? See the example of Hudson Yards in Brooklyn:

Hudson Yards is the largest private development project in U.S. history, and it’s being built without footings or foundations. Instead, the project is going to sit atop 300 concrete-sleeved, steel caissons jammed deep into the underlying bedrock. Work on the platform broke ground last week, and will take roughly two and a half years to complete. In that time, there’s a lot of engineering to do.

Caissons are a technology borrowed from bridge building, and they are what makes this project possible. The engineers will drill them anywhere from 40 to 80 feet into the Manhattan schist (the dense, metamorphic bedrock that supports the city’s soaring skyline). The caissons are meticulously arranged in the narrow spaces between the tracks. Above, the they will connect to deep-girdle trusses – some up to 8 stories tall – that control and redirect the towering weight overhead. Finally, the slab. “The total punishment is somewhere in the neighborhood of 35,000 tons of steel and 50,000 cubic yards of concrete,” says Jim White, the lead platform engineer. And that’s before they start loading buildings on top.

Building an elevated platform over an active train yard requires clockwork scheduling. White used computer models to coordinate the tempo of his drilling and truss-laying around the rhythm of the rails. “We look at the area of the yard and model in the train traffic, when it moves on an hourly basis and actually design the connections so we can install these 100 foot long trusses when have a window of opportunity,” says White. For the two and a half years it will take to complete the platform, there are only four scheduled track closures.

That is quite a lot of weight over a rail yard. However, such projects are not unknown in large cities where people look to maximize both space above and below ground. Space is at a premium so construction projects need to get creative and allow for a multitude of uses.