NEW YORK — 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.

Before 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.

“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. “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.