Anyone who’s been stuck on BART in the Transbay Tube waiting for a train to inch forward assumes that a second tube could help prevent those annoying delays.

Public officials and riders have talked for years about building a second Transbay Tube — someday. Now, with BART’s ridership soaring, trains more crowded than ever and the economy booming, the idea is getting serious attention.

Last month, San Francisco Mayor Ed Lee said he will begin a “regional conversation” with transit officials and other mayors about building a second tube to land at rapidly developing Mission Bay. Supervisor Scott Wiener, who is also a member of the regional Metropolitan Transportation Commission, recently said a second tube is needed to provide 24-hour service. And at a BART Board of Directors workshop Thursday, Alameda’s public works director said his city is eager to work with the transit agency on a new tube to bring service to his island city.

But despite the surge of interest, a new tube won’t arrive as quickly as anyone would like. It would take many years — perhaps 30 or more — to build political support, satisfy environmental concerns, decide where it should go, come up with many billions of dollars, and finally, build the new line.

“If anyone wants to know how long it it will take and how complicated it will be, think about the eastern span of the Bay Bridge or high-speed rail or plans to extend runways at SFO,” said Martin Wachs, a retired UC Berkeley planning professor who specialized in transportation and is a senior researcher for Rand Corp. in Santa Monica.

Passengers demanding more

A second tube would give BART, and its 400,000 daily riders, some relief. It would create a way to get around broken-down trains or other troubles in the tube. It could allow the transit system to run round-the-clock service, now precluded by maintenance needs.

Most important, it would increase the capacity of the system, which is becoming increasingly stressed by ridership that has grown much faster than BART anticipated.

“We’ve got a system that’s hurting and customers who are demanding more and more from the system,” said Grace Crunican, BART’s general manager.

BART’s just-released biennial customer survey show evidence of that pain: Riders’ overall satisfaction with the transit system, while still relatively high, dropped by 10 percentage points — from 84 percent in 2012 to 74 percent in 2014.

That’s the lowest satisfaction rating since 1988, said Aaron Weinstein, BART’s chief marketing officer. Riders were most dissatisfied with crowded trains and stations, the inability to get a seat, and the cleanliness and conditions of the trains and stations. All of those are troubles that could be alleviated by increasing capacity.

BART has ideas to boost its capacity, but no real plan for a new tube exists. Aside from some drawings and brief descriptions in a 2007 regional rail study that could be labeled visionary, dreamy or wishful, depending on your perspective, a second Transbay Tube is little more than an idea.

“There’s not much there,” BART spokeswoman Alicia Trost said.

Tube is 'basically full’

The agency’s priority, Trost said, is finding ways to add capacity and cope with overcrowding — but in a time frame much shorter than 30 years. Those include running longer trains or extra service during commutes, finding funding to expand BART’s fleet to 1,000 rail cars from its current 669, building additional platforms at the congested Montgomery and Embarcadero stations in downtown San Francisco, or adding crossover and storage tracks at critical points in the system. BART also wants to replace its train control system, allowing it to run trains closer together, and build a new maintenance center so it can service trains more quickly.

“It’s a big deal that the mayor has gotten squarely behind having a second BART tube,” Wiener, the San Francisco supervisor who has made transit one of his signature issues, said recently. “Having a second BART tube isn’t optional. We have to do this. The tube is the BART system, and it’s basically full.”

A new tube, at this point, is an unfocused vision, far in the distance. A study of a new tube isn’t likely to start until at least 2017, when a recently initiated study of transit capacity in the Bay Area transit core — essentially the Transbay Tube, Bay Bridge and Market Street subway — is completed. The study by the Metropolitan Transportation Commission will recommend ways to boost capacity on all transit systems that move through the corridors — not just BART.

“That study will help determine how we move forward,” Trost said.

An unfocused vision is still a vision, and ideas tossed out recently see more than a mere twin to the existing tube. A new one might head from Oakland to Alameda then to San Francisco’s burgeoning Mission Bay neighborhood and up to Market Street, with potential lines heading out Geary Boulevard to Ocean Beach, north to the Presidio or south to the city’s southeastern areas. How travel times might be affected would depend on where a new tube and lines were located.

The 2007 regional rail plan suggested a stop at Oakland’s Jack London Square and routes carrying BART into South of Market, where passengers could transfer to the Central Subway. Earlier visions had also pondered connecting Oakland and San Francisco international airports.

Major funding challenges

Deciding where a tube and its connections might go is just part of the process. It would need to satisfy environmental concerns about the bay, wetlands and wildlife as well as neighborhoods it would pass through.

But the biggest challenge would be finding money to pay for it — and building the political and public support to persuade the state and federal governments, Bay Area taxpayers and possibly developers to contribute billions for the project.

Few cost estimates exist, and transit experts are reluctant to speculate because details don’t exist. But the regional rail plan suggested a $10 billion cost — for the tube and connection to Market Street only. BART officials think the cost would be closer to $12 billion — almost twice as much as it cost to build the new Bay Bridge eastern span, a project that took 16 years from design selection to opening day.

Jim Wunderman, chief executive officer of the Bay Area Council, a regional business advocacy group that pushed for the creation of BART more than 50 years ago, said he believes it’s possible to put together plans to build and pay for a new tube without waiting three decades.

“I don’t think 30 years is reasonable to look at a project that has the ability to improve the quality of life so substantially,” he said.

Wunderman said the Bay Area has long supported taxes to improve transportation and that BART has been successful in persuading the state and federal governments to give generous grants for major projects.

Regardless of that generosity, voters probably would have to shoulder much of the cost of a new tube and expensive expansions of the BART system on either end, said Gabriel Metcalf, executive director of SPUR, an urban think tank in San Francisco.

Investment in future

The Bay Area has failed for decades to invest in public transportation, aside from suburban BART extensions, and it’s time to pay up, Metcalf said.

“We’re going to have to tax ourselves, and we’re going to have to make some investments for the future,” he said. “And, hopefully, we can find some inspiration around the idea that we need to leave this region better off than it was when we showed up. The generation that got BART built did that for us. Now we need to do that for the generations that come behind us. We are a very wealthy region, we can definitely afford to do this.”

That would help Charlyne Hardy, an East Bay commuter who works in San Francisco and rides BART through the Transbay Tube.

“It’s frustrating,” she said of getting stuck in the tube. “It creates anxiety. You’re stuck there, you look for an exit, but you can’t get out.”

Michael Cabanatuan is a San Francisco Chronicle staff writer. E-mail: mcabanatuan@sfchronicle.com Twitter: @ctuan