(BCN) — BART and the Santa Clara Valley Transportation Authority are on track to begin construction of BART’s downtown San Jose extension by 2024, officials with the two transit agencies said Friday.

Design and construction officials with BART and the VTA said in a joint meeting that tunnel boring and other heavy station construction activities are likely to start in earnest within the next two years, barring unforeseen delays. The six-mile, four station extension will add four new stations at 28th Street in San Jose’s Little Portugal, near the intersection of Santa Clara and Market streets in downtown San Jose, the San Jose Diridon rail depot and a ground-level station near PayPal Park in Santa Clara.

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The extension will include more than four miles of rail tunnel under downtown San Jose along Santa Clara Street. BART and VTA officials plan to bore under the city to construct the tunnel rather than cut open Santa Clara Street to build the tunnel and then reseal the roadway, a method known as “cut and cover.”

“What we deeply care about is building the best project, the most equitable project ever that connects the Bay Area and our communities together in a way that we would have never believed possible,” said BART Board Director Janice Li.

Under previous design proposals, the subterranean BART stations would be encased in a single tunnel bore, with the platforms for north and southbound trains stacked on top of each other. However, officials said on Friday that the proposed subterranean station designs will now feature a single platform sandwiched between parallel tracks for trains traveling in either direction, a design similar to BART stations like that of Embarcadero station in San Francisco.

The modification – suggested by the project’s design build contractor Kiewit Shea Traylor Joint Venture, or KST – would widen the proposed tunnel bore by roughly five feet and is expected to be easier to maintain, cheaper to construct and more efficient to access both for riders and emergency responders. The single platform is also expected to improve ventilation for the tunnel, according to Takis Salpeas, the project director for the San Jose extension, because it will include a 13-foot by 17-foot cavity under the tracks.

“You can operate a pick-up truck underneath there,” Salpeas said. “People can go under there and work if necessary.” BART, VTA and other South Bay officials christened the first phase of the South Bay extension in June 2020, with the opening of BART stations in northern San Jose’s Berryessa neighborhood and Milpitas.

The San Jose and Santa Clara extension, which will constitute the second phase, is expected to cost roughly $6.8 billion and be largely completed by 2028. Funding to complete the project is expected to come in part from federal grants, either from the U.S. Federal Transit Administration or other sources within the U.S. Department of Transportation.

Testing and certification for the extension is estimated to take approximately two years once construction is completed. Passenger service through San Jose would start in 2030 on the current timeline.

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