NEW YORK (AP) — The first piece of a nearly $4 billion redevelopment of the World Trade Center transportation hub debuted Thursday with the official opening of an underground concourse that passes through an area that has been closed since 9/11.

The gleaming, marble-paved expanse is expected to smooth the way for tens of thousands of commuters and visitors. It ultimately will feature retail outlets, but it offers something new right now: A passageway that links businesses and ferry service to the west of the trade center site to New Jersey-bound PATH trains and the rest of lower Manhattan to the east.

Prior to Sept. 11, pedestrians used a bridge over heavily traveled West Street. Since the attacks destroyed the bridge, they've used a temporary bridge or crossed the streets at street level. The temporary bridge is being dismantled and is not in use.

"The original World Trade Center site eliminated the street grid because that was the fashion of the times," Port Authority of New York and New Jersey executive director Patrick Foye said at Thursday's ribbon cutting. "This restores that street grid and adds an underground grid that literally spans the length of lower Manhattan."

Foye noted that designing the $3.9 billion transportation hub, scheduled to be completed in 2015, provided the opportunity for a "do-over" of sorts that focuses more on linking multiple modes of transportation than the original World Trade Center site did.

The hub will connect the PATH rail system, ferry service, New York City subway lines and the Fulton Street Transit Center. Gone will be the days, Foye said, of commuters having to cross busy streets and trudge up and down stairs to make transit connections, Foye said.

The approximately 600-foot-long underground concourse, which features 40,000 square feet of Italian marble, will house stores and restaurants on two levels, also by 2015. The Port Authority is partnering with Westfield Group to develop and lease the more than 350,000 square feet of retail space. Westfield had signed a long-term retail deal with the Port Authority not long before Sept. 11 and signed a new deal for the redeveloped site in early 2008.

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Other components of the redeveloped World Trade Center site will be rolled out over the next several months.

The 72-story 4 World Trade Center is scheduled to open next month, and One World Trade Center, once known as the Freedom Tower, is expected to have its official opening in early 2014.

The first new PATH rail platform to replace the temporary platforms that have been used since Sept. 11 should open by the end of this year or early in 2014, Steven Plate, World Trade Center construction director, said.

Yards from where hurrying commuters passed through the temporary PATH station Thursday, workers continued the construction of the massive, 800,000-square-foot transportation hub, whose dominant feature will be an "oculus," two wing-like sections of arches separated by a huge skylight.

"To use a football analogy, we feel like we're on the 20-yard line and we're about to punch it in," Plate said.