BART won’t go to Livermore, the transit agency’s Board or Directors decided after a nearly four-hour hearing in Oakland on Thursday.

On a 5-4 vote, the board sided with those who felt BART should focus on rebuilding and modernizing the existing system before it commits to building more extensions. Directors Bevan Dufty, Nick Josefowitz, Rebecca Saltzman, Lateefah Simon and Robert Raburn voted against building the $1.6 billion extension. John McPartland, Debora Allen, Joel Keller and Tom Blalock voted for the extension.

Supporters of the Livermore extension, who packed the BART board room, seemed surprised by the vote — and disappointed.

“The Bay Area lost an opportunity to provide a regional rail transit connection that would have benefited tens of thousands of drivers stuck on Interstate 580,” said Livermore Mayor John Marchand. “The commuters of the Bay Area lost on this one.”

The board also voted against proceeding with plans for a rapid bus system that would have directly connected to the Dublin/Pleasanton Station after Livermore residents said they weren’t at all interested in the idea.

But the decisions may not be the end of the line for those wanting a transit connection from Livermore to Dublin/Pleasanton BART. The Tri-Valley-San Joaquin Valley Regional Rail Authority, created by state legislation last year, could come up with its own plans, and financing, for a rail connection between the Altamont Corridor Express commuter train, which stops in Livermore, and BART.

The extent of the authority’s power is uncertain, but, as of July 1, it can begin working on a connection to BART. Could that include a conventional BART extension?

“I don’t know that we can build a BART extension,” said Alameda County Supervisor Scott Haggerty, a member of the authority board. “But there are precedents. Look what San Jose did. But there are other options as well.”

He was referring to the Santa Clara Valley Transportation Authority’s arrangement to build its own BART extension and pay BART to operate it.

While Livermore residents have clamored for a BART extension for decades, the board was considering not only lengthening the existing system, it was also contemplating options for smaller trains, a rapid bus system and a plan to speed up existing bus service.

But, in the end, it chose to focus on upgrading its current system.

More than 150 people, most of them backing what they called “the full BART” option, filled the transit district’s Oakland board room and spilled over into an adjoining room where they watched the proceedings on a big-screen TV as if it were the Warriors’ playoff game they were missing.

Proponents, most of them Livermore residents, argued that the extension would lure commuters off of I-580 as well as encourage development of jobs and housing in the Tri-Valley region. Critics of the extension said BART needs to instead spend its money on rejuvenating the existing system and expanding its capacity. They encouraged the board to instead back the rapid bus option, which would have a direct link to the Dublin/Pleasanton Station.

The plan for a conventional extension would have taken the BART tracks down the I-580 median to Isabel Avenue in Livermore, where city officials recently approved a plan to develop 4,100 housing units and offices for 9,100 jobs. Without a full-fledged BART extension, city officials argued, those jobs and homes will not materialize.

Several speakers said that BART owed Livermore an extension since residents had been paying sales and property taxes to the transit district for 50 years.

But the majority of the board was apparently persuaded by those who argued that BART needs to clean up, speed up and modernize the existing system and plan for things like a second Transbay Tube.

“I just couldn’t vote for this project,” Dufty said of the extension. “because of the effect it would have on the system.”

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com