SAN JOSE — BART service to downtown San Jose — including the crucial stops at the Diridon train station and First Street — could slip to as late as 2030 under some new estimates being floated by the Valley Transportation Authority.

At one point, political and business leaders had anticipated BART service beginning in 2026 in downtown San Jose, but the new estimates from VTA point to a service launch more in the 2029 or 2030 time frame, according to the transit agency’s BART project.

“It would be fair to say that we are looking at a construction timetable that would be later than the original schedule from 2014,” said Bernice Alaniz, BART Silicon Valley business operations and communications director.

BART is deemed a crucial propellant that would help fuel development and corporate expansions in downtown San Jose.

“This is a preliminary estimate,” Alaniz said, referring to the view that service may not start until 2029 or 2030. Alaniz added, “If nothing else changes, the time could get pushed out” for the start of BART service in downtown San Jose.

The current estimates are based on a design endeavor that is roughly 2 percent complete, Alaniz noted. As a result, those estimates could be adjusted further — including a time frame that’s shorter than 2030 — as the design work proceeds.

“Given the complexity of this, no one expected this to happen in 2026,” said Scott Knies, director of the San Jose Downtown Association. “But to hear 2030, you realize that’s a decade from now. Now, the best thing is to get on this and make sure it doesn’t slip any further.”

The rapid transit system is expected to provide stops at First and Santa Clara streets in the core of downtown San Jose as well at Diridon Station on the western edges of downtown.

Google is planning a transit-oriented community of office buildings, homes, shops, restaurants, cultural amenities, hotel rooms and open spaces near Diridon Station and the SAP entertainment and sports center.

Diridon Station already is a hub for light rail, Amtrak, Caltrain, ACE Train and the Capitol Corridor lines. It’s also touted as a future stop for BART and high-speed rail links.

The reasons for the new estimates for BART service, as of now? VTA cites multiple factors.

For one thing, environmental clearance had been anticipated in 2017 but was pushed back to 2018. Then, to help minimize disruption to merchants along Santa Clara Street, beneath which BART trains would run, VTA spent additional time to craft a single-bore tunnel option for BART’s approval. The single-bore approach and size of the tunnel mean that construction crews would be able to tunnel roughly 30 feet a day beneath downtown San Jose.

All those factors could portend a later start than previously anticipated for the first BART trains to begin running through downtown San Jose, Alaniz said Tuesday.

It’s possible VTA and BART could discover fresh ways to package the construction and other components of the project in a fashion that could speed things up and achieve service prior to the current preliminary reckoning of 2029 or 2030.

The potential delay could offer an opportunity for greater efficiencies between VTA and BART in the view of Teresa Alvarado, director of the San Jose office of SPUR, a nonprofit planning organization. Alvarado also is one of the members of a VTA-organized community working group monitoring the downtown San Jose BART project.

“This demonstrates the challenges of two major transit agencies working together, which is what we need more of in the future,” Alvarado said. “Going it alone isn’t what we want. We need seamless transit in the Bay Area and for transit agencies to learn how to integrate their services.”

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“2030 is not that much of an extension, and it will go by in the blink of an eye,” Alvarado said. “We have to make sure that the underground segment beneath downtown is done right. We have to be sure this is done in a way that does not disrupt all that we plan to build in downtown San Jose.”