BART’s long-awaited extension through downtown San Jose, meant to serve a new Google campus with up to 25,000 employees, is poised to get a $125 million windfall from the federal government to bring its construction plans into reality.

The Federal Transit Administration picked Valley Transportation Authority — the Santa Clara County agency that is funding and building the BART San Jose extension — for a pilot program to fast-track funding for major transportation projects. VTA requested about $1.4 billion in federal funding for four new stations and a track that will loop west and then veer north to wind up in Santa Clara, as well as other infrastructure. The agency will collect the $125 million grant announced Wednesday when it meets all the requirements to proceed to a construction agreement.

Once the project is complete in 2026, officials estimate it will carry 52,000 riders each weekday and cut back 27 million miles of car travel annually by encouraging people to ride mass transit instead. It’s also expected to reduce vehicle fuel emissions by 22,340 metric tons each year.

Transportation officials with the VTA and the federal agency joined city and state leaders Wednesday under the awning of the North San Jose sign of the Berryessa Transit Center in San Jose to announce the grant.

San Jose Mayor Sam Liccardo said Silicon Valley suffers from a “suburban sprawling pattern,” where families struggle to find affordable housing and residents are caught in traffic that is “choking us.” The extension project will be instrumental in creating a promising vision of Silicon Valley transit for everyone to take advantage of, Liccardo said.

“I want to note that this $125 million is really a down payment on the future,” Liccardo said. “And of course, this is a down payment on the future of transit.”

The new BART route stood out in part because much of its 6-mile track from San Jose’s Berryessa neighborhood through downtown and into Santa Clara goes underground, using a single-bore construction method in which trains run side-by-side with a center platform.

That design spared downtown San Jose from invasive construction that would tear up the streets.

“The reason that Google has long publicly stated that they’re coming to downtown San Jose is because of the transformational impact that our BART extension will have for the downtown area,” said Carl Guardino, president and CEO of the Silicon Valley Leadership Group and a key booster of the project. The group led campaigns for sales tax measures in 2000, 2008 and 2016 to help fund the South Bay BART extension, as well as the Regional Measure 3 bridge toll hikes, some of which will go toward the 10 miles of new track.

Nuria Fernandez, the CEO and general manager of VTA, said it is the first in the nation chosen for the Federal Transit Administration’s expedited project delivery program.

“VTA is demonstrating the merits of this transformational infrastructure project being the first in the nation to use single-bore technology for transit,” Fernandez said.

An innovative tunnel concept wasn’t the project’s only selling point. VTA also demonstrated that it could cover at least 75% of the costs with local and state funds, as well as public-private partnerships. And it created potential for a plethora of new transit-oriented development, with homes and offices clustered around transit stations to help lure people out of cars.

“This project is one that is very innovative and so we are very excited to move to the next level with VTA on that project,” Federal Transit Administration Acting Administrator K. Jane Williams said in a statement.

In addition to the Google campus at Diridon Station, VTA has plans in the works for “non-car-dependent communities” near the future Alum Rock/ 28th Street Station and the terminus in Santa Clara, said project spokeswoman Bernice Alaniz.

VTA officials expect to open the first segment of the line, 4 miles of track with two stations in Milpitas and the Berryessa district, by the end of this year.

San Jose Rep. Zoe Lofgren credited fellow Democratic Reps. Ro Khanna of Fremont, Anna Eshoo of Palo Alto and Barbara Lee of Oakland, among others, for supporting the project, calling them all “crucial in moving this forward.”

“About half the jobs that were created in California have been created right here in Silicon Valley. That’s an amazing fact, and it creates prosperity, but it also creates the challenges of prosperity,” Lofgren said. “With this project now viable, we can see solutions on the way.”

Those challenges, Lofgren said, involve affordable housing and traffic congestion, two issues she says are connected. Lofgren also thanked the “wise voters” living in Santa Clara County who “understand that there is no free lunch” and that “no challenge is beyond our capacity to beat it.”

Note: An earlier version of this story mischaracterized how trains would be oriented in the tunnel beneath San Jose. They would run side-by-side in a single bore with a center platform.

Rachel Swan and Lauren Hernández are San Francisco Chronicle staff writers. Email: rswan@sfchronicle.com, lauren.hernandez@sfchronicle.com