The coolest walk in San Francisco isn’t on the street.

It’s under the street.

For all the fuss and griping about traffic problems caused by the Central Subway construction, it’s hard not to be impressed when you’re 100 feet down in the futuristic, 20-foot-high concrete tube.

When Central Subway Program Director John Funghi offered a tour of the subterranean railway route this week, I quickly accepted. If nothing else, when the 1½-mile subway route opens, I can say I walked the rail bed when it was just a cement wormhole below the surface.

Funghi has been proselytizing about the virtues of the subway for more than 10 years, just after the Municipal Transportation Agency put the first shovel in the ground.

“You should be able to go from the Caltrain Station at Fourth and King to Chinatown in seven minutes,” he said.

Which frankly sounds awfully optimistic. After all, the trains will be above ground at the Caltrain Station and will roll up Fourth Street amid the typical SoMa traffic for a block and a half before diving underground. And, given the swarms of people who line curbs on Third Street every morning waiting for a bus to Chinatown, it seems unlikely that a short-run subway would have much effect.

But after walking deep into the tunnel, it all starts to make sense. The bores create a traffic- and stoplight-free corridor where trains can cruise unimpeded at 35 mph. Although the Central line stops at Chinatown now, the tunnel goes all the way to the old Pagoda Theater, and once people see how efficient this spur is, I’ll bet the call to extend it to North Beach will be emphatic.

More important, this is a part of the inevitable shift away from automobiles in the city. Not because of an ideological shift, but because it is becoming too damn hard to navigate the streets.

As San Francisco Municipal Transportation Agency Director Ed Reiskin says, this isn’t a new concept. Way back in 1973, the Board of Supervisors designated San Francisco as a transit-first city. Back then it was a concept. Now it’s a necessity.

“As the second-most-densely populated city in the country (New York City is first), I think our roadways are nearly at capacity,” said Reiskin. “And our streets are not going to get any wider.”

It isn’t just San Francisco. The American Public Transportation Association, based in Washington, has data that says in 2014 annual ridership on public transportation was the highest in 58 years.

Other urban centers are also going underground. Los Angeles is at work on three underground projects, including a 3.92-mile underground railway extension under Wilshire Boulevard. Seattle is in the process of constructing a 3.1-mile underground route that will reach the University of Washington from downtown.

Back to Gallery Central Subway project walking tour shows light at end of... 4 1 of 4 Photo: Paul Chinn, The Chronicle 2 of 4 Photo: Paul Chinn, The Chronicle 3 of 4 Photo: Paul Chinn, The Chronicle 4 of 4 Photo: Paul Chinn, The Chronicle







New York City has two projects under construction, including a 2.3-mile stretch in downtown Manhattan. It’s starting to look like subways are the new freeways for commuters.

“What we have seen in many older cities around the world is that subways are the most efficient,” Reiskin said. “Now Los Angeles, in particular, is doing a lot of subway work. I think once we see the difference between a subway and trying to operate on the surface, there will be support.”

In San Francisco, the two tunnels, north and south, look virtually ready to go right now. But Funghi says there’s still plenty to do before the 2019 opening. A rail bed has to be laid and infrastructure like fiber-optic cables have to be installed.

There were those (OK, I was one) who scoffed at the short length of the route, but once you’re in the tube, it seems vast. In fact, Funghi says he took a news service reporter on a tour and she got a little freaked out.

“She turned ghost white,” Funghi said. “and she said ‘Can we go out? I don’t feel safe.’”

That probably happened about the time Funghi told her that because the tunnels are below San Francisco’s water table, if a leak formed in the concrete wall, water would shoot out at a force of 30 pounds per square inch.

“We’re completely submerged,” he said. “We’re under water right now.”

Which, of course, raises the question of earthquakes. Funghi has all the right answers about seismic structural integrity, but adds two other points.

“The beauty of this is that it doesn’t cross any fault lines,” he said. “And it is always safer to be underground in an earthquake because there’s nothing to fall on you.”

Except the ceiling, of course. So there may be commuters who find the underground experience creepy, but the feeling is, when people see how well the subway works, it will be seen as a great alternative.

Reiskin says he knows one thing for certain.

“By 2019, when we cut the ribbon,” he said. “Traffic will not have gotten any better.”

C.W. Nevius is a San Francisco Chronicle columnist. His columns appear Tuesday, Thursday and Saturday. E-mail: cwnevius@sfchronicle.com Twitter: @cwnevius