SAN JOSE — In a “huge victory” for downtown businesses, BART and Santa Clara Valley Transportation Authority staff are set to recommend building one tunnel instead of two for the long-awaited San Jose BART extension when their boards meet next month to discuss the multi-billion-dollar project.

Both agencies recently reached an agreement on the divisive issue, according to a letter BART General Manager Grace Crunican sent to Bay Area legislators Wednesday. BART initially favored twin bores for safety reasons, while the VTA wanted a single bore because it would cause less disruption to downtown and cost less to build.

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“VTA and BART have been working collaboratively for several years to move this vision forward and have come to the point where staff of each agency will recommend to their respective Board of Directors that a single bore alternative be adopted,” Crunican wrote.

The VTA board is slated to take up the recommendation on April 5 and the BART board on April 26. Spokespeople for each agency declined to comment on the agreement and how it was reached ahead of the meetings.

The boards could break new ground if they choose to go with a single bore. The tunneling technique hasn’t been used before with passenger railroads in the United States.

“I think it’s a reflection of honest effort on both sides to recognize transit construction design needs innovative approaches,” said Mayor Sam Liccardo about the agreement. “We’ve seen considerable progress in recent weeks to find a path forward that ensures our subway is both safe and compatible” with a rapidly developing city.

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In her letter, Crunican said VTA General Manager Nuria Fernandez “deserves both credit and praise for getting us to this pivotal point.”

“Through her leadership and determination we trust that, if approved, the agencies will be able to build and operate an extension that will result in years of safe and efficient transit service,” she continued. “We look forward to a celebration with you down the road where we can all appreciate her hard work and dedication to increasing public transit opportunities in our impacted communities.”

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The San Jose Downtown Association heralded the agreement as a “huge victory” for it and the entire downtown community.

“BART will be dug as a single bore tunnel, keeping the construction mostly below Santa Clara Street,” the association said in a Facebook post Friday. “Great to have solidarity on this issue.”

In a twin-tunnel design, the tunneling machine bores underground until it gets to the station areas, where crews then begin the difficult work of tearing up the street and relocating the utility networks underneath them, a process called “cut and cover” that almost always leads to delays.

By building a single, 4.7-mile tunnel deep underground, the VTA says it can shorten an otherwise 5-year construction timeline by 10 months. The project would also make BART an industry leader once again.

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BART cited safety concerns in its previous support of the twin tunnel option, which is used elsewhere in its system and throughout the country. A different design would require the agency to train its operators and instruct its passengers on up to three different evacuation procedures — variations that, in an emergency, could lead to a moment’s hesitation or a simple mistake that BART officials say might make the difference between life or death.

The boards are up against a deadline to make a decision on what kind of tunnel to build. To get funding and start construction on the estimated $4.7 billion extension in the next two years, the VTA and BART boards must settle on an option at their upcoming meetings.

Staff writer Erin Baldassari contributed to this report.