SAN JOSE — When it’s completed, the four-station BART extension into downtown San Jose and Santa Clara will be the most significant public transit investment the Bay Area has seen in decades.

Not only will it link the economic powerhouse of Silicon Valley to the East Bay and San Francisco, it will also connect BART to Caltrain and high-speed rail, transforming San Jose into a regional transit hub unlike any the Bay Area has ever seen.

To get there, both BART and the Santa Clara Valley Transportation Authority (VTA), which is funding the construction and operating costs of the extension, need to agree on several potentially contentious issues. The most pressing is how to build the tunnel connecting the soon-to-be completed Berryessa BART station with four new stations to be built in downtown San Jose and Santa Clara. By 2035, 52,000 passengers every weekday are expected to ride BART trains on the new extension.

Both agencies say a deal is imminent, but time is running short.

“The engineers are still working furiously on solutions,” said San Jose Mayor Sam Liccardo. “I’m seeing progress I hadn’t seen in months.”

The VTA says it can shave 10 months off construction by building a single, 4.7-mile tunnel deep underground. The design would be a first in the United States and requires technology Liccardo is hoping the city will pioneer. The project would not only put BART back on the map as an industry leader, but it would spare San Jose’s downtown businesses years of disruptive construction just as the city is coming into its own as a hotbed for commercial and residential construction.

Citing safety concerns, BART favors the same twin tunnel design used elsewhere in its system and throughout the U.S. A different tunnel design would require BART to train its operators and instruct its passengers on up to three different evacuation procedures — variations that, in a fire or another emergency, could lead to a moment’s hesitation or a simple mistake that agency officials say might make the difference between life or death.

(Click here, if you are unable to view this video on your mobile device.)

Gas prices on the rise, officials rushing to approve BART extension to San Jose and the Bay Bridge Series will now have a trophy are some today’s Hot List stories.

“It’s almost like they are talking past each other,” said Phillippe Thomas, the chief engineer at the Metropolitan Atlanta Rapid Transit Authority, who was briefed on the project last fall when BART and the VTA asked him and five other experts to help them evaluate their options. “They each have a view based on their experience, and somehow you have to blend both viewpoints to make the system work.”

And there’s roughly $1.5 billion in federal grant funding on the line, about one-third of the estimated $4.7 billion needed to complete the project.

To get funding and start construction in the next two years, the VTA’s governing board needs to make its decision by April 5, and BART must approve that recommendation by April 26.

Why one tunnel?

In true Silicon Valley style, the VTA is proposing an innovative tunneling technique that hasn’t been used before with passenger railroads in the United States, though it has been used to build a transit tunnel in Barcelona.

Rather than having a center platform flanked by trains running in opposite directions, as BART has in downtown San Francisco, station platforms in a single-tunnel design are stacked one on top of each other. That means there is less space for packed crowds during peak commute times, and it’s harder to transfer passengers from a disabled train to a working one.

The big benefit is that the single tunnel would be much deeper underground and the boring machine can dig below the surface without massive street-level disruptions. But it also means longer escalators and taller elevator shafts, which are more costly to operate and hard to repair. Difficulties with repairs already plague BART’s aging system; a national shortage of repair workers even prompted the agency to create its own apprenticeship program.

In a twin tunnel design, the tunneling machine bores underground until it gets to the station areas, where crews then begin the arduous work of ripping up the street and relocating the complex utility networks underneath them, a process called “cut and cover” that almost inevitably leads to delays.

The most recent example of twin tunnel construction in the Bay Area is Muni Metro’s Central Subway, a four-station, 1.7-mile extension of its light rail line through Union Square and Chinatown in San Francisco. The project is now a year behind schedule, said Paul Rose, a spokesman for Muni.

Utility relocation was a big part of that, said Cheryl Brinkman, the chairwoman of Muni’s governing board.

“Utilities are something that are just invisible to all of us,” she said. “We don’t know what’s going on under our streets.”

For downtown San Jose business owners who survived the VTA’s light rail construction some 30 years ago, the prospect of another huge construction project eating up parking spaces and making a mess of the street is disconcerting.

Alfredo Diaz, of Diaz Menswear, used to be located on the VTA light rail route and remembers not only the grumbling customer complaints, but the neighboring shops that were forced to close.

Now he’s located a few blocks away on East Santa Clara Street between 2nd and 3rd streets, right where the new downtown San Jose BART station is planned.

“Most of our business is people who walk by or drive by,” he said. “(The construction) is really going to make all of our customers debate whether they want to come see us or go someplace else.”

The impacts of the station construction in the twin-tunnel design are expected to last up to five years and will be predominantly focused on a relatively small area: the three-and-a-half blocks along Santa Clara Street between Market and Fourth streets in downtown San Jose, said Grace Crunican, BART’s general manager. The Alum Rock station is located under an industrial area, the Diridon station will be built under a parking lot, and the Santa Clara station will be built above ground.

“It’s not the length of Mission Street or the length of Market (Street),” Crunican said, referring to BART’s original construction through downtown San Francisco in the 1960s, which tore open a more than mile-long hole down Market Street, decimating surrounding businesses.

So is one tunnel really unsafe?

If this was an entirely new system, the different style of station design wouldn’t be a problem, said Paul Oversier, BART’s assistant general manager for operations. But this will be an extension of an existing system, and that’s where trouble starts.

In the single tunnel design, the trains would move on tracks that sit side-by-side in the tunnels until they approach stations, when one set of tracks would braid over the other. If a fire started on the upper level, passengers would have to access a stairwell that leads them deeper underground, an escape route that’s counter-intuitive to most people, said Robert Solomon, a division manager at the National Fire Protection Association.

In the fog of a life-threatening situation, panic sets in, and even the best trained employees make mistakes, said Jeff Keating, the rail transit committee chair for the American Railway Engineering and Maintenance-of-Way Association. Consistency is one way to help prevent those mistakes.

“It’s not that people can’t be trained, but from a practical standpoint, it causes more problems,” Keating said, adding that the risk also extends to maintenance workers. “All you need is a second of inattention and then you’re dead.”

Nor is it clear that VTA could save the time it claims with a single tunnel. Designs for the twin tunnels and related stations are further along, meaning their costs are more certain. And, using one machine, instead of two, means there’s a lot more risk of delays if something happens to the machine, said Gary Brierley, the president of Doctor Mole Incorporated, a tunnel engineering firm.

He pointed to Seattle’s “Big Bertha,” a massive machine that dug a 57.5 foot-diameter tunnel under the city’s downtown core for a new highway. When the machine broke down, construction stopped, and the project is now four years behind schedule.

When it comes to long-term investments that last a century or more, it’s the day-to-day experience that make a difference to the riders, he said.

“Everybody complains about construction … but San Jose has to think about the value of this investment in the long-term, and they need to think about what people want,” Brierley said. “They want a system that operates reliably and consistently, and they want a system that’s safe.”