The long-awaited BART extension to San Jose’s urban core will involve one tunnel instead of two—a big win for downtown businesses.

A letter sent last week from BART General Manager Grace Crunican to regional legislators recommends single-bore because it would save money and temper the impact of building massive new infrastructure in the heart of the city.

The Santa Clara Valley Transportation Authority (VTA) and BART, which have been working for two decades on the multi-billion-dollar project to stretch the train line from Fremont to central San Jose, will bring the single-bore proposal to their respective boards of directors, the letter states. VTA’s board plans to discuss the recommendation on Thursday; BART will later this month.

The question of whether to opt for a single or double bore tunneling was a divisive one, with BART initially pushing for the latter option because it’s consistent with infrastructure design elsewhere in the region. But locals feared a repeat of the upheaval from the VTA’s light rail construction in the 1980s, which wreaked so much havoc that it spawned the San Jose Downtown Association to advocate for embattled stakeholders.

A rendering of the single-bore design, which delves deeper underground than its double-bore counterpart. (Source: VTA)

The double-bore design would feature parallel channels linked by a passenger platform. It would lie closer to the surface than a single-bore tunnel—about 35 feet below ground as opposed to 85 feet—and would require an estimated $70 million more to construct.

A single-bore tunnel, by contrast, would cost less to build because it would bypass major public utility infrastructure. But it would cost slightly more to operate over the long-term. The VTA puts the price of running a single-tunnel system about 2.8 percent higher over a 30-year period than the two-bore alternative.

Building a two-tunnel system would not only cause significantly more surface-level disruptions, it risks a political toll as well by potentially undermining public support for a project that’s already behind schedule and over budget.

In a recent Mercury News op-ed advocating for single-bore, downtown Councilman Raul Peralez said he saw how debilitating it was for local businesses when construction problems pushed back the completion date of VTA’s new express bus line along the Alum Rock corridor. The impact of a far bigger project through a far denser part of the city would be all the more devastating, he noted.

“I cannot begin to fathom the damage a 40-foot deep by 1,400-foot long void would cause, just so we could accommodate twin-bore tunnels, when really we could achieve the same safe result with single-bore,” Peralez wrote. “Those who have invested time, talent and treasure into our downtown deserve better.”

The one-tunnel design would set new precedent, officials said. Single-bore tunneling hasn’t been used before with passenger trains in the U.S., BART and VTA officials said. According to urban planning think tank SPUR, the only transit system in the world that uses a single-bore tunnel is Barcelona’s Metro 9 Line.

BART’s top administrator applauded her counterpart in the South Bay for coming to an agreement on the tunnel design.

“VTA General Manager Nuria Fernandez deserves both credit and praise for getting us to this pivotal point while maintaining a commitment to both safety and speedy project delivery,” Crunican wrote in her March 28 letter. “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.”