Some Long Island Rail Road commuters were in for quite a surprise when they stepped off their trains in Manhattan and walked to the west — an alternative to Penn Station.

At 6 a.m. Thursday, the doors opened for the first time on a new customer concourse at the Farley Post Office building, connecting to the Penn Station tracks.

The new concourse, along with new entrances and exits on Eighth Avenue, mark the completion of the first phase of the long-awaited Moynihan Station.

The 400-foot-by-400-foot concourse provides direct access to 17 of Penn’s 21 tracks, including all the tracks used by LIRR trains. The concourse is connected to the westernmost end of the platforms.

The first commuters exiting their trains Thursday morning, largely construction workers, emerged into a concourse that is bright, spacious and modern — adjectives not typically used when describing the cramped, dingy and dated LIRR concourse at Penn Station.

“It’s bright and it’s clean,” said Al Merkle, 52, of Ronkonkoma, as he took in his new surroundings with wide eyes. “This place is usually horrible.”

The new space expands on an existing concourse that was narrow and dimly lit, and did not have direct access to the street level. The new concourse features several massive, high-definition digital screens, including just above the escalators, that feature schedule and track information and images of New York.

New signs are large and bold, directing riders to the LIRR, Amtrak and NJ Transit, as well as to buses and cabs at street level. Yellow portals direct riders down onto tracks, and keep crowds from forming at the concourse waiting area. And slanted, glass windows provide commuters a rare glimpse of the track level from the concourse above.

The new concourse also features digital skylights that create the illusion of blue skies and soft clouds overhead despite the low ceilings.

“It’s beautiful. . . It makes it seem bright. You come through Penn Station and it’s kind of dark down there,” said Lynne Watson, of Flushing, who works in customer service at the post office just above the new concourse. “It definitely makes you not feel like you’re underground.”

Watson was coming up the stairs from her train platform when workers opened the gates to reveal the new space. Until Wednesday, the new concourse, and Eighth Avenue entrances, were blocked from view by makeshift walls.

The Empire State Development Corp., which is overseeing the Moynihan project, began work on its $300 million first phase in 2012. Gov. Andrew M. Cuomo has said he expedited the project to have the new concourse open in time for the expected LIRR service disruptions that are necessitated by Amtrak’s planned track work at Penn this summer.

The new space will help disperse potential riders and allow the LIRR to better handle the crowds that are expected, including from trains that will be lengthened with additional train cars.

The state is expected to close with developers Thursday on the $1.6 billion second phase of the Moynihan project, which will include a 300,000-square-foot new train hall with another concourse for LIRR customers, retail and a 92-foot-high skylight roof. That phase is expected to be finished by late 2020.

The new train hall was initially proposed to be used exclusively by Amtrak, which owns Penn, but Cuomo directed the agency and developers to include the LIRR in its plans last year.

The project will also come with renovations to the existing Penn Station, including wider walkways and taller ceilings at the LIRR level.