Hundreds of workers are busy around the clock building the nearly $4 billion World Trade Center transportation hub, its white steel wings starting to rise into the Manhattan sky as a remembrance to those who died at the site on 9/11.

Designed by Spanish architect Santiago Calatrava, the complex will be “a light-radiating work of art” when it opens sometime in 2015, trade center construction director Steven Plate said during a tour Wednesday.

Reaching three levels underground, the 800,000-square-foot complex will link the PATH trains with Hudson River ferries to New Jersey, and 11 city subway lines — all already operating.

Under construction are the two wing-like sections to be separated by a special feature: a skylight that will open like a huge eye. On each Sept. 11, this “oculus” will be aligned so direct sunlight will shine through the glass at 10:28 a.m. — the time when the second tower collapsed in 2001.

His inspiration for the design, Calatrava once said, was a child releasing a bird.

“This is more than an engineering marvel,” said Plate. It’s designed “to remember the souls of nearly 3,000 people who died here.”

Towering over the hub is the 104-story 1 World Trade Center, the centerpiece of the rebuilt 16-acre trade center site. And right above the complex lies the 9/11 Memorial plaza with its reflecting pools.

The $3.9 billion cost of the steel-concrete-and-glass structure falls into a budget under the Federal Transit Administration and the Port Authority of New York and New Jersey that owns the trade center site.

Plate compared the hub’s 150-foot high main hall to that of Grand Central Terminal, with 100,000 square feet of upscale retail space planned to serve about 250,000 World Trade Center commuters passing through each day.

More than 12,000 tons of steel is needed to complete the complex, with up to 500 people working there in any given 24-hour period.

On Wednesday, they braved a heat index that hit 100 to keep the labyrinth of construction equipment, scaffolding and cranes going.

Brows dripping under hardhats and huge safety glasses, the workers sweated in heavy-duty gloves and steel-reinforced boots.

“We’re drinking a lot of water, down under by the tracks, in and out of the sun all day — very hot,” said carpenter Elizabeth Fontanez. “We make sure we have gallons of water all around to stay hydrated.”

The 42-year-old Bronx resident said she changes shirts several times during her 10-hour shift.

Still, with a tool belt, hammer and harness weighing at least 20 pounds strapped to her waist, “we take little breaks here and there.”

But “I love my job” — rebuilding the World Trade Center.

Among the engineering challenges was the destruction wreaked by Superstorm Sandy last October, Plate said.

Salty water flooded the lower level, destroying electrical conduits and half-built escalators that had to be entirely replaced.

But, said the civil engineer, “I see resilience here after 9/11 — and resilience after Sandy.”

Photo Credit: In this file photo of July 11, 2013, construction cranes work inside the elliptical shape of the World Trade Center's transportation hub in New York. The complex will include PATH trains to and from New Jersey, and links to half a dozen city subway lines when it opens sometime in 2015. Mark Lennihan / Associated Press