A correction to an earlier version of this article has been appended to the end of the article.

The plans for bringing BART to downtown San Jose are audacious. Trains will travel through the biggest subway tunnel ever built, large enough to fit a five-story building or a four-lane freeway, for nearly five miles through the heart of the city. When passengers arrive, they will step onto station platforms 100 feet below street level — so far down riders will have to be whisked to the surface in high-speed elevators.

To the $5.6 billion project’s boosters and VTA, which is building the extension that BART will eventually operate in the unique arrangement that brings the system to Santa Clara County, it’s an innovative plan befitting the region it will serve.

“We’re Silicon Valley, we can do things the rest of the world doesn’t do,” said Rod Diridon Sr., the longtime transit advocate for whom Diridon Station, the site of a future BART stop, is named.

But to its skeptics, who have watched the project’s price tag balloon and its opening date slip — while VTA and BART struggle to complete a far more conventional extension to San Jose’s north side — the downtown project’s immensity and complexity are red flags.

“There really is no advantage to reinventing the wheel,” said Jonathan English, an urban planning doctoral candidate at Columbia University who studies big transportation projects like the BART extension. “They are trying something new that is really risky.”

VTA’s plans for the project call for using a nearly 56-foot-wide tunnel-boring machine — which would be among the largest ever built — to dig the 4.7-mile length of the San Jose subway. That would create a 50-foot-wide tunnel large enough to accommodate two BART trains, separated by a common platform, all within a single tube.

Construction on the extension is scheduled to start in 2022. The tunnel-boring machine is expected to burrow under San Jose for several years.

Pioneered by subway engineers in Spain, the single-bore design has become increasingly popular because it allows transit agencies to build underground rail lines without disrupting life on the street above. San Jose businesses and city leaders had balked at the idea of ripping up downtown streets for years at a time while the extension is built, which the more conventional “cut-and-cover” subway designs would require. And so, after an intensive study that included a visit to a Spanish rail line that uses the design, VTA officials chose the single-bore plan 2018.

But that wasn’t the end of it. The design VTA picked was a 45-foot-wide tunnel, which would have required the underground station to have stacked platforms for each direction of travel — similar to how BART’s Downtown Oakland stations have north- and south-bound trains on different levels.

BART opposed that idea, insisting the San Jose stations needed to have a single platform, like most of the rest of its system, to make emergency evacuations simpler. So last year, VTA agreed to widen the proposed tunnel and allow for a center platform between the two tracks.

“It just was not a feasible scenario for us,” BART Assistant General Manager Carl Holmes said of the stacked design. “We were looking out for the best interest of our riders.”

The difference might not seem like much, but think back to geometry class: The area of a 56-foot-wide circle is about 50 percent bigger than that of a 45-foot circle, meaning the project will require a much bigger boring machine, which will have to remove much more earth from beneath San Jose.

The resulting tunnel will be far bigger than the 40-foot-wide Spanish bores that inspired it, and nearly as large as the biggest tunnels bored for any purpose.

“You’re talking about doing something that is just off the charts, in terms of skill level, from anything that has been done regionally in a long, long time,” said Elizabeth Alexis, a co-founder of the group Californians Advocating Responsible Rail Design. Alexis is concerned the novel design could make delays or cost overruns more likely.

One of VTA officials’ main selling points to the public for the single-bore design in 2018 was that it would be cheaper and quicker to build than typical methods, which require boring two smaller holes. But that promise may not hold up.

Last year, VTA officials increased the extension’s estimated cost by $900 million and pushed back their timeline for completing it by as much as four years — from a $4.7 billion project finished in 2026 to a $5.6 billion one that could open in 2029 or 2030.

VTA spokeswoman Bernice Alaniz said it was too early in the design process to say if the wider tunnel is substantially more expensive and time-consuming than other methods, and attributed the higher price tag for the project to the rising cost of construction and real estate. Alexis and English, the doctoral candidate, warned the larger tunnel design could wipe out any savings from boring one tunnel instead of two.

Colorado School of Mines tunneling professor Michael Mooney was not concerned about VTA’s plans for the subway and doubted the bigger bore would mean a much bigger price tag.

“When you get up in that range, the differences between a few feet in diameter don’t make a big difference in terms of the overall cost,” Mooney said. “I would challenge the notion that the large bore is riskier.”

There are also questions about plans for the three underground stations VTA would ultimately build as part of the project. With platforms 10 stories below street level, they would be nearly twice as deep as the platform at BART’s Embarcadero station, which is 54 feet beneath the surface.

Alaniz said the depth means that high-speed elevators are the fastest and most efficient way to move people to the platform at the planned downtown station near Market and Santa Clara streets, and at an Alum Rock station near where Santa Clara crosses Highway 101.

The BART stop at Diridon station will need to have escalators because of the big crowds it will handle after events at nearby SAP Center, Alaniz said. The extension’s last stop in Santa Clara will be above ground.

A handful of far-below-ground subway stations in the United States use elevators to whisk passengers to and from their platforms, but the design is unprecedented in the BART system, where elevators don’t exactly enjoy a sterling reputation for cleanliness or reliability.

While plans for the tunnel and stations are complex, Alaniz said VTA has not heard concerns about them from prospective contractors.

And despite repeated delays with the first phase of the BART extension to the Milpitas and Berryessa stations, which are still not open, Alaniz defended the agency’s ability to deliver big infrastructure projects.

“We were on schedule — we were actually ahead of schedule” in building the two stations and 10 miles of track, Alaniz said, but testing problems and the discovery that one of VTA’s contractors installed improper communications equipment have forced major delays.

VTA has hired Takis Salpeas, a Virginia-based consultant with 35 years of experience building subway projects around the world, to manage the BART extension. Under his contract, VTA will pay Salpeas up to $3.2 million over five years.

“He knows what he is charged to do,” Alaniz said. “He was brought specifically to deliver Phase I and get Phase II moving,” she said, referring to the Berryessa and Downtown extensions.

Perhaps the biggest question, though, is whether BART and VTA will work together effectively, or if the extension’s extra layer of complexity — having one public transit agency build an extension that another will use — will further bog down the complex downtown project.

Officials from both agencies acknowledge that arrangement has caused problems as they try to wrap up the $2.3 billion Berryessa extension, though they insist the next phase of the project will go better.

Alexis said the push and pull over the tunnel design shows the two agencies are still not on the same page.

“Maybe (they) have learned something, but it doesn’t feel that way so far,” Alexis said.