S.F. leaders tour completed Central Subway tunnel

People on a tour walk through the northbound tunnel of the Central Subway on Monday, May 18, 2015 in San Francisco, Calif. People on a tour walk through the northbound tunnel of the Central Subway on Monday, May 18, 2015 in San Francisco, Calif. Photo: Lea Suzuki, The Chronicle Buy photo Photo: Lea Suzuki, The Chronicle Image 1 of / 59 Caption Close S.F. leaders tour completed Central Subway tunnel 1 / 59 Back to Gallery

The future of transit in San Francisco was exposed Monday, and though it was dim, gray and dusty, city leaders and transportation officials said it portends a bright future with less traffic congestion.

Mayor Ed Lee took a brief walk into one of the two Central Subway tunnels, which were officially completed last week. The mayor and other city officials strolled a couple of blocks beneath Fourth Street to where crews are excavating the Yerba Buena/Moscone Station, where construction crews have yet to break into the subway itself.

Standing where northbound trains are scheduled to run starting in 2019, the mayor said the 1.7-mile subway would not only speed transit travel but relieve congestion. Traffic South of Market, especially near the Bay Bridge, is slowed by multiple construction projects, including the Central Subway. He also brought up the possibility of an extension to Fisherman’s Wharf.

Video: Inside the Central Subway

“We are well on our way to opening this subway up in 2019 and having a concrete solution to congestion in this part of the city,” Lee said.

Whether or not the subway relieves traffic remains to be seen. What is certain is that the twin tunnels definitely involve a lot of concrete. The circular tunnels, 20 feet in diameter, each stretch 8,500 feet — from beneath the Interstate 80 skyway at Fourth Street to Chinatown.

Two tunnel boring machines — named Mom Chung and Big Alma after prominent women in San Francisco history — completed the excavation work last summer, placing 1,750 concrete rings that comprise the wall as they went.

The machines were extracted from a giant hole in North Beach in June, and the finishing touches were applied, including construction of the southern portal, where trains will enter and leave the tunnels.

The $1.6 billion subway project, which will become part of the T-Third Muni Metro line, extends from Fourth and King streets, near the Caltrain terminal, above ground to a station at Fourth and Brannan streets before heading underground near Harrison Street. It passes beneath Fourth and Stockton streets to the end-of-the-line station at Washington Street. Trains will stop at subterranean stations at Moscone Center, Union Square and Chinatown.

The tunnels continue to North Beach. City officials and some North Beach residents are hopeful the line will eventually be extended to that neighborhood and possibly all the way to Fisherman’s Wharf.

Lee voiced his support for an extension during a subterranean news conference.

“It is done so well,” he said, “that I think there is tremendous interest to take it all the way to Fisherman’s Wharf. I hope to see that at least sometime in our lifetime.”

Inside the subway tunnel, it’s hard to imagine you’re beneath the busy streets and tall buildings of downtown San Francisco. It’s gray, mostly dry and quiet, especially the farther you get from the ventilation fans. Entering from the portal, the tunnel is straight, heading slightly uphill as it passes Moscone Center and heads downtown. At Market Street, it dips beneath the BART and Muni Metro subways, then climbs uphill again toward the Union Square station.

John Funghi, Central Subway project manager, said one tunnel was completed in eight months, the other in 11, finishing the job about a month ahead of schedule and slightly under budget. Construction of all of the stations is under way.

“We’ve made amazing progress,” he said.

Next, a contractor will install a floor to the tunnels, put down tracks and erect communications systems and overhead power wires.

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

Online extra

To see a video from inside the Central Subway tunnel, go to: http://bit.ly/1ETBwcF.