MILPITAS — It will mark the first time BART has crossed into a new county since 1973, but riders looking forward to the openings of the Milpitas and Berryessa stations will have to wait a little bit longer for the historic moment.

Earlier this year, officials said they were ahead of schedule and anticipated opening the new Santa Clara County leg before the end of 2017. On Tuesday, Valley Transportation Authority spokeswoman Brandi Childress said that’s not happening — the project is now on track to open in June 2018.

“Once we got into the system testing phase and got an update from the contractors on the progress, we had to resequence some of the events,” she said at a media briefing and tour at the Milpitas station. “We are talking June for passenger service, that is the goal. We are still on time and under budget.”

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Part of the adjustment in schedule stems from a change in how the new Warm Springs BART station operates. It needs to be converted from an end-of-line station to a pass-through operation.

BART spokesman Jim Allison said the trains need to be able to operate autonomously to maintain a schedule, and right now they’re set to start and stop their runs in Warm Springs.

“Adding 10 miles means the train control operations must be changed,” Allison said. “It involves verifying all the logic for the train controls.”

Allison said that officials will be conducting tests along the line on weekends in September and October, shutting the Warm Springs station down as the test trains are put through the motions. And sometime next month, they’re going to run a train in manual mode, operated by a driver, down the tracks and into Santa Clara County.

The last time a new county line was crossed was when BART nosed just over the San Mateo County border and into the Daly City station in 1973.

“It’s been a long, long time,” said Allison.

The new stations are expected to be a boon, with projections of 20,000 new passengers served at the Milpitas station and 25,000 at the Berryessa station by 2030. Both stations, expected to open at the same time, include large six- and seven-story neighboring parking garages. The Milpitas station will also be a transit hub, with a new VTA line going to Mountain View and most bus lines being shifted from the Great Mall transit station to the new site.

VTA Berryessa project manager John Engstrom, who led a Tuesday tour of the Milpitas station, said the architecture is more open and features art glass and skylights that can be seen even from the underground platforms.

“It’s light and airy,” Engstrom said. “You prefer natural light coming in than having fluorescent lights. Natural sunlight helps give a sense of openness.”

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Allison said the type of problems that plagued the opening of the Warm Springs Station, 5.4 miles south of the longtime end-of-the-line Fremont Station, should not repeat on the next leg. Warm Springs opened two years later than officials had hoped.

“The delays we had involved getting new software to match up with the existing track, and we have ironed out a lot of the details,” Allison said. “We don’t expect to repeat those problems all the time, and have to rewrite code to make sure it’s up to our specifications.”

He said another delay at Warm Springs involved an old cable that carried more than 34,000 volts to power the system that “we kept splicing and splicing, and it finally failed.”

“That should not be an issue,” he said. “This is all new.”

The new stops are part of a 10-mile extension into the South Bay that will eventually go underground and through downtown San Jose to Santa Clara for an additional 6 miles. That won’t be done until 2026, Childress said.

Engstrom said that despite the additional time, there isn’t much cost involved because it’s not construction.

“It’s the testing,” he said. “The elements don’t involve a huge budget — like construction — but they do take time. I compare it to having an electrical short in a car; it’s likely going to take weeks to find the problem but five minutes to fix.”

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The Berryessa project broke ground in April 2012. Childress said construction will be completed by the end of the year, but testing the system will extend until summer.

“The community will appreciate that it has to be safe, it has to be reliable, and we’re not going to open it up for service until that happens,” she said.