You will know them by their unmistakable lights: In less than a year, from summer 1972 to summer 1973, three enormous projects that reshaped San Francisco reached completion. As Watergate gathered steam and the hippie era flamed out, William Pereira’s 853-foot Transamerica Pyramid became an instant landmark, the tallest building west of Chicago. At some point shortly after, Sutro Tower poked its alien-probe head above the marine layer for the first time — and on July 4, 1973, it began broadcasting TV service to neighborhoods like Cole Valley that had made do with spotty reception, essentially ever since Philo Farnsworth invented television on Green Street in 1928.

In between the unveiling of the city’s two tallest manmade structures, on Sept. 11, 1972, Bay Area Rapid Transit began service from MacArthur Station to Fremont. Criticized for low initial ridership and for going over budget, construction nonetheless plowed ahead, and the Transbay Tube opened in 1974, restoring a rail link between San Francisco and Oakland that had vanished when the Key System ceased running streetcars on the Bay Bridge in 1958. Only 15 years later, BART proved its vitality when a section of the Bridge collapsed after the October 1989 Loma Prieta earthquake. Upon inspection, the system was found to be virtually undamaged, and the Transbay Tube remained open 24-hours-a-day for weeks as ridership jumped 50 percent.

But that bout of heroism was nearly 30 years ago. The nine-county Bay Area has since grown by more than a quarter, to 7.7 million people. Collectively, the four urbanized counties BART currently connects — San Francisco, San Mateo, Alameda, and Contra Costa — have grown by more than 1 million people over that timeframe, and the region could have upward of 9 million people by 2040. The transportation network is already strained, and while we can add ferry terminals and implement contraflow bus-only lanes on the Bay Bridge during peak times, we can’t widen the bridge itself.

BART’s mildly horrifying, fabric-covered seats have all been replaced, but overcrowded trains and routine delays due to various components reaching the end of their lifespan contribute to a sense that the system is in serious decline or even a money pit. The debut of the “Fleet of the Future” trains was pushed back several times. “Systems integration” issues held up the Fremont line’s extension to Warm Springs, as getting a 1970s computer to communicate with a 2010s computer proved challenging. (Accommodating bird migrations cost money, too.) On the morning this story went to press, “smoldering debris” on the tracks at 24th Street caused systemwide delays, with 20-minute wait times during rush hour.

But rail transit is entering a renaissance of sorts in California. Caltrain electrification and the state high-speed rail project are moving ahead, and even in notoriously BART-phobic Marin, you can now ride from San Rafael to Santa Rosa on SMART. While the Trump administration’s grand, trillion-dollar infrastructure plan might amount to nothing, the stars are coming into a alignment for a second Transbay Tube.

California is the freeway state, the promised land of cheeseburgers and low-density homes and all the trappings of postwar suburbia. Yet the reaction against the dominance of private automobiles has slowly gathered strength to the point that when Sen. Dianne Feinstein and Rep. Mark DeSaulnier floated a proposal for a new bridge that would combine vehicular traffic and BART, the reaction was harsh and swift. Streetsblog wrote, “The 1950s called: They want their transportation policy back.”

There have been calls since the 1940s for a so-called Southern Crossing, most likely from Hunters Point to Alameda, and Feinstein has advocated such a span before. In the 2000s, two studies concluded that such a project would likely cost $12 billion, about double the $6.4 billion cost of the Bay Bridge’s new eastern span and roughly in line with the projected $12 to $15 billion price tag of a second Transbay Tube. Sometimes, rail plans get bundled into the idea of another bridge, which may or may not be greenwashing. But the salient issue isn’t cost. It’s distribution: Where would those thousands upon thousands of cars go?

As Los Angeles learned during the I-405 widening, better known as Carmageddeon, vehicular traffic operates along the “law of induced demand.” That is, if you build it, they will come — and they will quickly overcrowd the roads so that you’re basically right where you were before.

San Francisco has no appetite for road-building. Decades after Malvina Reynolds strummed “Cement Octopus” in the Panhandle, the ensuing freeway revolt has halted further freeway construction for the foreseeable future. If anything, I-280 in San Francisco is as likely to come down altogether as get beefed up for a flood of new traffic every morning, much of it single-occupancy cars destined for already-congested surface streets.

So transit advocates have stepped up their call for a second tube. In the last few years, official planners have indicated that such an undertaking may be necessary for the region by as soon as 2030, when demand outstrips carrying capacity. (The Metropolitan Transportation Commission’s 80-page “Bay Area Core Capacity Transit Study,” published last September, confirms this date.)

In April 2016, Tilly Chang, executive director of the San Francisco County Transportation Authority, moderated a talk at SPUR outlining the challenges such an engineering megaproject would entail, along with some possibilities for how it could be integrated into BART’s existing system. Wonks find these questions tantalizing: Would the tunnel be bored or would sections of it be lowered into the Bay, the way the original Transbay Tube was built? And where would the tunnel’s touchdown points be?

There are at least five options under study for where a tunnel might land. Sixteenth and Third streets in Mission Bay could work, serving the future Warriors arena and connecting to Caltrain. So might Mission and First, to sync with the Transbay Terminal and bleed pressure off BART’s trunk line, one block north.

Once in San Francisco, trains would have to go somewhere. One enticing possibility entails a Geary Boulevard line that hooks south down 19th Avenue and reconnects with BART at Daly City Station by way of San Francisco State, while another calls for a spur to the Richmond from approximately Geary and Masonic with a full line running to UCSF Parnassus and West Portal before connecting at Daly City. Whatever the ultimate route, Chang considers a second tube “essential for connectivity, capacity, late-night service, and resilience.”

The timeline is another matter. Ten years might not be feasible, but 2040 is certainly too late — especially for downtown San Francisco. Upzoning in SoMa will introduce tens of thousands of new residents to the area, many of whom will have 20-minute walks across numerous four-lane thoroughfares to reach any existing Caltrain or BART station. The Downtown Extension, which involves running Caltrain another 1.3 miles from Fourth and King streets to the Transbay Terminal at First and Mission streets, is all but certain — but it’s insufficient, as Embarcadero and Montgomery stations are already overcrowded. So the MTC has several proposals, including new BART tracks down Third Street or Brannan Street, as well as a second Transbay Tube.

Having passed Measure RR in 2016 with 70 percent of the vote, BART won approval for a $3.5 billion bond issue dedicated toward various system upgrades, demonstrating that the electorate hungers for better transit. This June, voters in the entire nine-county area will vote on Regional Measure 3, a massive, $4.45 billion suite of capital projects that encompasses everything from additional regional bus service and ferry terminals to the extensions of Caltrain to the Transbay Terminal and BART to San Jose, plus improvements to the Richmond-San Rafael Bridge and an upgraded Clipper system and many other things.

There’s something in RM 3 for everyone in every county, including $50 million allocated toward evaluating a second Transbay Tube. Admittedly, that’s a pittance, less than half of 1 percent of the eventual cost. But future funding doesn’t have to come from taxes and fees, nor does it have to be distributed by a federal government that seems to be back-tracking on its commitment to public works. Something of this scale happens only once a generation or so: Regional Measure 1 was in 1988, and Regional Measure 2 in 2004.

As Chang notes, “Cities need to be creative in order to do this type of big-picture infrastructure planning. … Anything you can have a revenue stream from, you can bond off of.”

It’s worked in the past, but it requires a comprehensive plan beyond an elementary engineering study. Ellen Smith, BART’s acting manager for strategic and policy planning, notes that present studies of future transit needs and conceptualizations of the engineering required to make them happen won’t be finished until at least the third quarter of 2021.

“Everyone wants to start with drawing a line on a map,” she says of the transit-wonk fantasy of Geary extensions. “But the hard work is trying to figure out where the future trips need to go and what form of transportation can best serve those trips.”

Smith cautions that even if built, a second Transbay tube might carry standard gauge as well as BART gauge. Rail gauge refers to the width of the tracks, and BART’s mesures 5 feet, 6 inches. That’s bigger than standard gauge — which is four feet, eight-and-a-half inches, on which Caltrain, Amtrak, and almost every other major rail system in the U.S. run — by almost a foot.

Why BART chose uniqueness over standardization is unclear. It may be because 1960s planners were simply enthralled with their Space Age technology, or because they wanted to run trains on curves at high-speeds (and over the windy Golden Gate Bridge). It may be because a gentleman’s agreement of sorts kept BART from intruding onto the formerly powerful Southern Pacific Railroad’s right-of-way. It may be because no one had built a new subway system in half a century, so the wisdom of adhering to standard gauge was lost in a spirit of postwar utopian hubris.

Whatever the reason, we’re stuck with what former 20-year BART Board member and current Livable City executive director Tom Radulovich calls “a fascinating anomaly,” best characterized by its “non-interoperability” with other transit systems.

“The folks who designed the system didn’t really know what they were doing, but they had a great deal of faith in themselves,” Radulovich says. “The tube was designed to work perfectly all the time. There’s not a lot of redundancy in the system. Very few pocket tracks, crossovers, turnouts — the things you’d do if a train goes out of service and you need to get it out of the way, or single-track around an obstruction, or you lose power. When BART goes down, it goes down big.”

Another curiosity regarding BART’s genesis was that it occurred right as urban rail transit in the U.S. hit its nadir. Downtown business elites were persuaded that to retain San Francisco’s importance as a central business district, there needed to be a way for white-collar workers in distant suburbs to get to work. Half a century later, it’s the main way they do so.

“Right now, two-thirds of the trips across the Bay, peak-hour, peak-direction, are on BART,” Ellen Smith says, adding that twice as many people commute by BART than by car. “We think that’s the pattern of the future, and we want to support the economy with that type of infrastructure.”

Forecasting future growth is a tricky business, but state demographers project it will be steady, with the current Bay Area effectively merging with Greater Sacramento and the San Joaquin Valley to become a sprawling, 20-county megalopolis. But if planners determine that future trips under the Bay will be better served by connecting Caltrain to an improved Capitol Corridor, then BART might have to wait.

Bureaucratic inertia can strangle good ideas in the cradle, but it can also give bad ideas unstoppable momentum. No politician wants to leave a bridge half-finished and dangling in the air, which leaves such projects vulnerable to incompetent managers or exploitation by shady contractors. Boston’s Big Dig required 16 years of active construction to bury a highway and build one bridge and one tunnel. It went 190 percent over budget, and some 56 years may elapse from initial planning (1982) to final repayment (2038). Farsightedness may require ordinary people tolerating the intolerable, as unelected planners placate irate taxpayers with empty buzzwords like “data-driven.”

No matter how necessary they may be, megaprojects generate opposition. Doctrinaire NIMBYs file suit, affluent citizens lobby lawmakers to make tweaks, eminent domain annihilates a neighborhood’s favorite mom-and-pop store, small-city mayors quarrel with regional leaders, and advocates point out how low-income residents will once again bear the brunt of the noise, the dust, and the disruption. In a city where laundromats may be historical and the removal of dying acacia trees from Van Ness Avenue necessitates public hearings, a second Transbay Tube will require sustained political leadership to see it through.

Nick Josefowitz, the vice president of BART’s nine-member board and the District 8 director, believes the answer is in deliberate foresight, and plenty of it.

“We’ve found ourselves in a situation where, over the past decade, the folks that have been elected to deliver on our needs for today and plan for our future have neither delivered on our needs today nor planned for our future,” he says over tea at a Blue Bottle in Pacific Heights. A second tunnel “is not going to happen in my political lifetime, but whether a project can happen in someone’s political lifetime should not be a metric we use to evaluate whether we do that project.”

Josefowitz, who is also running for San Francisco supervisor, ranks BART service on a second Transbay Tube as his No. 3 priority. Nos. 1 and 2 are a tie. He wants to upgrade Caltrain to connect to Oakland and Sacramento under the Bay, with frequent service and at high speeds, and also to “max out” BART’s existing infrastructure through other means, such as adding cars to the current trains and implementing a driverless train-control system that would enable 25 percent more trains to pass through the current tube at peak hours.

It’s less about avoiding rush-hour catastrophes and more about guiding the region to smarter growth, with housing density centered on stations.

“In my experience, there are hundreds of thousands of people who want to live within walking distance of a BART or Caltrain station,” he says. “And that’s the only way you make these pencil out, which is not as transportation projects but as regional projects.”

He reiterates Smith’s contention that you can’t just draw a line from A to B and then expect the pieces to fall into place, or else “you’re going to spend a lot of money connecting places that people don’t want to go to.” (This sounds like a swipe at high-speed rail, a troubled project that currently aims to link San Jose Diridon Station to Bakersfield by 2029 and parlay that into a San Francisco-Anaheim connection four years later. But when pressed, Josefowitz declines to specify.)

The other issue when planning for 2040 and beyond is what the nature of transportation will be. We can safely rule out flying cars — too many burrito-carrying drones clogging the airspace, no doubt — but autonomous vehicles loom. A self-driving Uber killed an Arizona pedestrian this week, and one death is too many, but 40,000 Americans die on the road every year. If, in five or 10 years’ time, autonomous vehicles demonstrate a vastly improved safety record over oopsie-prone Homo sapiens, it’s not inconceivable that we’ll live to see the mandatory phase-out of human drivers altogether. The difference between public and private transit, already blurred in the age of Lyft, might hybridize further. And between Tesla, SpaceX, the Hyperloop, and the Boring Company, Elon Musk is sure to bring about a change or two (even if at least some of those ideas go nowhere). To account for this uncertainty, Josefowitz thinks we need to prepare for multiple outcomes.

“We need to have a plan for if autonomous-vehicle adoption happens really quickly or really slowly, or if most of those autonomous vehicles are shared, or if everybody has their own,” he says. “There’s only a certain degree to which any one of us can influence the outcome, but we should have thought of all of these options so that we can make the best decisions today and we’re not blindsided in the future.”

Automation won’t just remove intoxicated drivers and people who text from the equation. It will also improve public transit. Right now, Josefowitz says, “the biggest cost in buses is bus operators.

“You have to run — infrequently, in a lot of places — big buses that are empty,” he says. “When ideally, we should be running smaller buses more frequently — which would be a lot more full, which would get you to more places. In San Francisco at night, Owl buses show up very infrequently because it’s really expensive, but if you could provide bus service at a similar cost with smaller vans arriving more often, you could imagine a more robust public-transit infrastructure.”

Right now, Chariot fills some of that, but not everyone in San Francisco can afford $5 rides at peak hours or even $3.80 off-peak. Indeed, Josefowitz notes, school attendance drops off toward the end of every calendar month as cash-strapped families find themselves unable to put kids on the bus. At the same time, eliminating municipal bus drivers would mean one fewer path to a middle-class standard of living for Americans without a college degree.

Other transit systems worldwide have advantages over BART. Moscow’s Soviet-era stations are magnificent, New York’s express and local tracks allow for 24-hour service on virtually all lines, and Mexico City’s rubber wheels permit a quieter ride than steel and greater track stability in a sinking city. Then there’s Shanghai. Because China is an authoritarian state with nothing remotely approaching CEQA or discretionary review, the government simply blasts and tunnels wherever it decides, and it opened 10 entire lines with hundreds of stations in less than a decade. One line connects to the world’s fastest train, a maglev to Shanghai Pudong International Airport, built 19 miles outside a city of 24 million. No American city will likely ever have something like that.

Questions of equity plague transit, even — or perhaps especially — in democracies. BART’s fare model, which charges riders more to travel greater distances, makes sense according to a strict miles-per-passenger calculus. It’s also palatable in a society where affluent, often white, suburbanites pay more than lower-income urban-dwellers, often people of color, who travel within the region’s core. But the 2010s population boom in central U.S. cities has upended that uneasy arrangement, and higher transit costs are an auxiliary injustice of displacement. Transportation-policy nonprofit TransForm addresses these concerns in a 2017 report called “Crossing Together — Equity Considerations for a Second Transbay Crossing.”

Displacement can be direct — knocking down your house to erect a ventilation shaft — or it can be indirect — making your neighborhood so desirable that you can’t live there anymore. Constructing new tunnels and stations tends to affect poor people quite visibly, but indirect displacement may be more insidious. It’s the carbon-monoxide poisoning to direct displacement’s fire.

West Oakland was devastated by three freeways, but BART cut through that neighborhood as well. With hope in our hearts, we might charitably assume urban planners have grown more sensitive to such concerns since Justin Herman decimated the Fillmore, but wealthy people simply have resources to get what they want and poor people do not. Noting that the San Francisco Bay Area holds the dubious honor of being the second-most-expensive urban agglomeration to build in — after New York — Josefowitz says he admires London’s Crossrail and Barcelona’s Metro for being affordable, environmentally sound, and cognizant of workers’ rights.

“They delivered them incredibly quickly and incredibly cost-effectively, and they’re going to see enormous ridership and benefits to their communities,” he says. “We have an enormous amount to learn.”

Encouraging dense development around transit stations in the midst of a housing crisis sounds like a strong positive, but it might accelerate gentrification. SB 827, a bill that would effectively upzone nearly all of San Francisco under the aegis of transit-oriented smart growth, has already become an issue in the mayoral race. Unlike Washington, D.C.’s Metro, BART doesn’t own much of the land surrounding its stations and track alignments, so it would have to partner with developers to accomplish anything at scale.

But extricating transit from its codependency with the automobile, particularly at suburban stations, has already begun. BART’s Ellen Smith is working to replace at-grade parking lots with transit-oriented development in places like Millbrae, Pleasant Hill, and Walnut Creek, part of an overall reconceptualization of BART over the last six or eight years, from endless expansion into distant exurbs and back to the region’s core. A second tube had been on the back burner for decades, with only sporadic internal discussions about how if you keep adding to the ends, you’re eventually going to have to add to the middle, too.

Now talk of it is shifting from “if” to “where.” Mission Bay is a potential landing site. A Dogpatch Neighborhood Association member with the intriguing name of Mc Allen — his Twitter photo is of the world’s lone triple-bore tunneling machine, used only once in Japan — thinks the intersection of Third and 16th streets holds promise.

“My grand vision for Third Street is that Caltrain and the high-speed rail move off its current right-of-way to go underneath Dogpatch and join Third Street. Then you can have a BART/Caltrain station co-located at 16th and Third, which would make a lot of sense,” McAllen says. “The core of downtown is very expensive to dig in already, so in terms of what is going on in Mission Bay, the Warriors arena will have peak demand that will be very high.”

Running a tunnel from there to the existing BART Station at 16th and Mission makes sense, although a dedicated bus lane in that corridor might be better, McAllen says. Radulovich is skeptical that downtown is moving south to Mission Bay at such a pace to warrant a BART line there, since most of the post-2008 high-rises have gone up closer to the Transbay Terminal.

“I think it’s a very debatable presumption, if you look at what’s built in Mission Bay,” he says. “It’s not Downtown; it’s Irvine.”

Were it to bypass Downtown, a second tunnel might be too far from “Big BART” to relieve overcrowding. The entire proposal might become something of a fetish object, unmoored from existing needs except when viewed from 35,000 feet — a classic pitfall for megaprojects. But to plan for 2040 means to take a guess at what San Francisco will look 22 years from now. Twenty-two years ago, in 1996, Salesforce didn’t exist. Now its name is on the tallest building in the city, plus the soon-to-open Transbay Terminal, and its annual conference takes over town for an entire week every fall.

“ ‘The Tube’ has become a rally cry if you’re a big thinker in transportation,” Radulovich says. “It’s now this emblem or signifier for bold-thinking individuals. I have a lot of questions about that: Where does it go? What system is it part of? What gauge is it? Those questions haven’t really been answered. The region could spend $20 billion on the tube, but it could be an answer in search of a question.”

Speaking on May 12, 1872, in Brooklyn, an Alameda County town long since annexed by the expanding city of Oakland, a visionary issued the following proclamation:

…We, Norton I, Dei gratia Emperor of the United States and Protector of Mexico, do hereby command the cities of Oakland and San Francisco to make an appropriation for paying the expense of a survey to determine the practicability of a tunnel under water; and if found practicable, that said tunnel be forthwith built for a railroad communication.

Just over a century would elapse before that vision came to pass, 35 years after the opening of the Bay Bridge (which Emperor Norton also issued several proclamations for). In that time, the internal-combustion engine came along, transforming a 30-mile trip from a day’s journey to a 20-minute jaunt to a soul-crushing commute for tens of millions of people.

Brilliant ideas for improving upon the ordinary, gasoline-powered car flamed out. Buckminster Fuller’s 1933 Dymaxion Car was a 20-foot-long, three-wheeled vehicle that could hold 11 passengers and travel up to 90 miles per hour at 30 miles per gallon. Only three were ever made. Like the Edsel Citation, GM’s all-electric EV1, the hydrogen-fueled dirigible, and the Concorde, the Dymaxion prototype is a museum piece awaiting a future that will never arrive. Great achievements in Getting from A to B have occurred, but no new solution for efficiently moving hordes of people into and out of cities at regular times twice a day ever arose. Ninety days after BART opened in 1972, human beings visited the Moon for the sixth time. We have yet to return.

A second tunnel will require a baton-relay of steady, technocratic hands to guide it through bursts of opposition, recessions, cost overruns, construction accidents, transfers of power between political parties, and the innumerable daily inconveniences that spur people to post Facebook status updates that consist of “Dear BART, Fuck you. Love, Me.” At no point, probably not until the first train that passes through it opens its doors to deposit speechifying dignitaries in some brand-new station, should a second Transbay Tube be considered a certainty. The individual who cuts the ribbon has not even entered politics yet. But whoever they are, they were probably stuck in traffic this morning.