Even as BART prepares to start service to Antioch this weekend and to connect to San Jose within months, the transit system’s leaders are considering another extension, this one to Livermore. But some BART officials wonder if the expansion would risk the health of the overall system.

During the 1990s and early 2000s, BART built extensions to Pittsburg-Bay Point, Dublin-Pleasanton, and San Francisco International Airport and Millbrae. What it didn’t spend money on was major maintenance.

And while some still dream of BART circling the bay and pushing into every populated crevice of the region, others say that vision is outdated and unaffordable, given the overwhelming need to modernize the overloaded and neglected system.

On Thursday night, the BART Board of Directors is expected to decide whether to inch forward with plans for a 5.5-mile extension of the Dublin-Pleasanton line to Livermore at an estimated cost of $1.6 billion. About $500 million has already been committed.

“I know what it’s like to have to drive over the hill to the BART station,” said Joel Ramos, policy director for TransForm, a Bay Area transit advocacy group, who grew up in Pittsburg when the nearest station was in Concord. “I know what it’s like to feel you were promised something. But that time of being able to build beyond our resources is long gone.”

Directors will decide whether to approve the environmental impact report for the project, and may or may not decide whether to proceed with the extension or choose an alternative transit approach that would most likely involve buses. The board may not make a final decision until June 14. An extension, which would be built in the median of Interstate 580, could take 10 to 15 years to complete.

The board is divided between those who favor the extension and believe it would serve commuters as well as encourage development of jobs and housing in the Tri-Valley region and those who say the agency needs to spend its money on rejuvenating the existing system and expanding its capacity.

The plan under consideration would extend the BART tracks to Isabel Avenue in Livermore, where city officials recently approved a plan to develop 4,100 housing units and offices for 9,100 jobs.

Bob Vinn, Livermore’s assistant city engineer and its BART project manager, said the development, both north and south of I-580, would feature four- to six-story condominiums and 1.7 million square feet of prime office space, a big change for a city of 89,000 known for its sprawling research labs, tract homes and wineries.

“None of that will happen without BART,” he said. “Livermore is not downtown San Francisco. We are a suburban community originally and are moving toward more of an urbanist type of development.”

However, urban planning organizations, transit advocates and some BART directors from denser urban areas don’t think Livermore, even with the development, warrants a full-fledged BART station.

They prefer either a diesel or electric train using traditional rails and smaller trains, an express bus line that would use I-580’s express lanes and connect directly to the Dublin-Pleasanton BART terminal or a spiffed-up version of existing local bus lines that would help them get to that station faster.

Despite the array of choices, only two — what Livermore residents call “conventional BART” and the bus rapid transit/express bus option are seriously under consideration.

In Livermore, Vinn notes, there is support for a traditional BART extension — and nothing else.

“There is very little support for express bus service,” Mayor John Marchand told the BART board at a meeting this month.

Livermore residents have clamored for a BART extension for decades, arguing they were promised a station in exchange for their support in the 1962 election to fund BART’s creation.

They also argue that they’ve been paying sales and property taxes that go to BART for more than 50 years. The environmental impact report states that Livermore residents have paid $201.6 million over the years, equivalent to $436.2 million when indexed for inflation.

Supporters say the extension is needed to take cars off of I-580 — often jammed with traffic in the morning and well into the evening — and support Livermore’s development goals. The environmental studies estimate the extension will add as many as 13,400 riders per day by 2040.

That’s not enough, critics say, to justify the cost.

“This project is way too expensive for the number of riders it’s going to carry,” said Nick Josefowitz, a BART director from San Francisco. “I think all of us really want to provide better transportation options to people stuck in traffic or struggling to get around the Bay Area, but the way we can do that is by investing in the best projects.”

Even after the passage of Measure RR in 2016, a $3.5 billion bond to fund rehabilitation of BART, the agency still needs about $4 billion to fix and modernize its infrastructure. That doesn’t include wish-list items like a second transbay crossing, estimated to cost a minimum of $12 billion.

State and federal transportation funding to pay for improvements ranging from track repairs to extensions is competitive, extension opponents say, meaning BART needs to set its priorities carefully.

“We have a lot of projects we need to fund, and there is not much transportation money out there,” said Director Rebecca Saltzman of Oakland. “We’re going to have to make a decision that considers the whole system, and we’re going to have to do that with this and every other (proposed) extension.”

But Livermore extension supporters say it shouldn’t be an either-or decision.

“We have a false choice when we say we support conventional BART or we support the core system improvements,” said Tim Sbranti, an aide to Rep. Eric Swalwell, D-Dublin, at a recent board meeting. “We can do both.”

The vote on whether to go to Livermore could be put off until BART’s June 14 meeting or a special session before July 1. That deadline was set in an obscure piece of 2017 state legislation that created the Tri-Valley-San Joaquin Valley Regional Rail Authority and gave it the authority to build a rail connection between the Altamont Corridor Express commuter rail trains, which stop in Livermore, from the Central Valley and BART.

It precludes the new authority from doing anything regarding a BART extension until July 1. If BART fails to act by then, the authority could take over the project — including finding funding — though it would ultimately require BART approval.

Should directors choose to go to Livermore, it’s not clear when the first trains would arrive. Livermore officials say the extension could be running by 2026, but BART officials say it could take at least that long to gather the needed $1.1 billion in funding.

“That could be,” Vinn said. “We’ve been waiting for 50 years and a lot of studies. We can wait another 10 to 15 years. If it doesn’t get approved, we get nothing.”

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