The idea of a southern rail crossing from the East Bay to Silicon Valley has enthralled politicians and transportation officials for decades. Yet with no money to back it up, it’s always been just a line on a map, tantalizingly snaking along the Dumbarton corridor.

But the winds shifted Wednesday night, during a town hall-style meeting at the Veterans Memorial Senior Center in Redwood City. About 100 people filled folding chairs in the auditorium, where representatives of Facebook, infrastructure developer Plenary Group and the San Mateo County Transit District introduced plans to change the future of transbay commuting by resurrecting a railroad of the past.

They’d start by rebuilding the Dumbarton Rail Bridge, a ragged, long-defunct structure just south of the busy car span. It’s the spine of an old Southern Pacific railway that officials are seeking to reactivate: A century ago, tracks on either side of the bay carried transcontinental freight and whisked passengers to vacation homes in Sunol.

“This is a once-in-a-generation opportunity to connect another part of the East Bay with the Peninsula,” said San Mateo County Supervisor Warren Slocum, his voice rising exuberantly as audience members leaned forward in their chairs.

The event — one of four held last week in various parts of the Bay Area — was unlike the typical spaghetti fundraisers that take place at the senior center. Business executives with plastic name tags greeted people who grazed at a Mediterranean appetizer spread; a PowerPoint slide showed the timeline for what is expected to be a billion-dollar project.

With an infusion of private investment, the Dumbarton Rail project could break ground in 2022. It may beat BART’s second Transbay Tube by a decade, or more.

“This is not a new, crazy idea — it’s something that policy advocates have wanted to do for a number of years,” Mark Evanoff, deputy city manager of Union City, told The Chronicle. He and others said the project had limped along since SamTrans purchased 18 miles of the right-of-way — including the bridge — in the 1990s, never amassing enough capital to see it through.

It wasn’t until 2016 , when Facebook chipped in $1 million for environmental studies, that the plan started to take shape. Though Facebook’s regional transportation chief Winsome Bowen was reticent about the financial details at Wednesday’s meeting, officials familiar with the project expect most of its funding to come from the private sector. Bridge toll revenue and a San Mateo County sales tax would supply a small portion.

“Facebook is pledging to build it, and maybe that’s all their money, or maybe they’ll ask other high-tech employers to contribute,” Evanoff said.

Worsening traffic conditions have forced Facebook into this new form of civic engagement. For years, the company has used private shuttles to zip people to and from work, a standard practice among tech giants. Now, Facebook has a whole division dedicated to transportation planning and is investing in transit that could affect the entire region.

Public-private partnerships were common in the 19th and early 20th centuries. Private companies funded the Western expansion of the railroads, and private investment paid to extend San Francisco’s Muni streetcar system to the avenues in the city’s western reaches , as a way to chase higher property values. But in the last century, transportation has shifted into the realm of public infrastructure, and the notion of a train funded by a company like Facebook is unorthodox.

“In a way, the Facebook train is an indictment of our inability as a society to keep up infrastructure needs,” said Randy Rentschler, legislative director of the Metropolitan Transportation Commission, which funds roads and transit throughout the nine-county Bay Area.

Though Rentschler supports the Dumbarton rail, he acknowledges that the model is controversial. Public-private deals have drawn criticism from people who say the government is shifting its responsibility onto a company, which then gets to call the shots. And Rentschler hopes they won’t become the new normal.

“A better option for our society is to get serious about what it means to build infrastructure in this country,” he said. “Absent that, we’re stuck doing things like this.”

If the financing structure seems pernicious to some people, the end product will provide benefits for both sides: Facebook gets more productivity from employees who aren’t sitting in traffic and the public gets congestion relief.

SamTrans CEO Jim Hartnett emphasized those points at the meeting Wednesday night.

“I don’t have to tell you about the congestion we all face, particularly those of us who use the Dumbarton car bridge,” Hartnett said, as members of the crowd solemnly nodded. Some were workers from the East Bay who spend long stretches of the day in snarled traffic; others hailed from neighboring suburbs where the streets are clogged with drivers escaping the freeway.

Supporters tout the railway as a potential Cadillac of east-west connections: The route that once carried freight could now serve as mass transit for tech employees who have fled the Peninsula for cheaper housing in the East Bay.

“Imagine what this missing link would mean for them,” Slocum said. “Imagine what it would mean for the environment.”

His speech drew smiles from some audience members, while others recoiled. A few who spoke out during a long question-and-answer session worried they would get all the headache of a new railway and none of the service.

“Most of the people in my neighborhood are opposed to this,” said Mike Morris, a longtime resident of North Fair Oaks, an unincorporated area between Redwood City and Atherton . He worries that the train would cut through their enclave of low-slung houses and brightly painted shops, bringing noise and the threat of accidents, while potentially lowering property values.

Yet others welcomed the plan and expressed hope that it would eventually expand. Officials want to stretch the track to the Union City BART Station, where it could connect with the regional backbone rail system, and one day link up with the Capitol Corridor trains traveling to Sacramento and Altamont Corridor Express trains heading into the Central Valley.

The prospect of a new regional hub captivated Adrian Brandt of Redwood City, who sat in the front row of the Veterans Memorial auditorium. He’s watched politicians fumble other plans, including an environmental study funded with 2004 bridge tolls that never saw the light of day. Officials with the Metropolitan Transportation Commission ultimately siphoned off those funds — nearly $135 million — for other projects, including the Warm Springs/South Fremont BART Station.

“I think it’s a reflection of misplaced priorities that this thing has been sitting and growing weeds while people are jammed up on (Highway) 101 and in Belle Haven,” he said, referring to a neighborhood that abuts Facebook’s headquarters.

While the rail dream languished, tech campuses sprouted from South Bay land once lined with fruit orchards and canneries. As jobs were created and housing growth flat-lined, the Bay Area’s east-west divide grew more pronounced. More people trekked daily from homes in the East Bay to jobs at Facebook in Menlo Park, Apple in Cupertino or Google in Mountain View, among others. Travel times across the Dumbarton span swelled from 25 minutes to over an hour during peak periods, and traffic spilled into city streets.

“Even before Facebook, we had this lack of connectivity,” said Union City Mayor Carol Dutra-Vernaci, who hopes the Dumbarton rail will solve congestion and other quality-of-life issues that vex her constituents.

Planners in Union City designed the neighborhood around the BART station with the expectation that trains would ultimately roll in. Over the last two decades, the city cleaned up more than 100 acres of formerly industrial land and filled it with housing and office space. Evanoff said he hopes the area will flourish once new train connections are solidified.

“This is an opportunity to connect Union City for both jobs and housing,” he said. “And then certainly people could go north on BART to Oakland and San Francisco.”

It could get even more elaborate, Evanoff said, imagining a double-tracked rail with trains zipping by in both directions, propelled by electricity , that maintain speeds of up to 79 mph.

It’s the vision that started gestating in the late 1980s, he said, but better.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan