Were it not for the subway, New York as it is today would not exist. At a crucial time in the city's history, the engineers of this ingenious subterranean railroad cleared the streets of impossible congestion and decanted the population of the teeming, insalubrious tenements of the Lower EastSide to the farthest corners of the boroughs. Because it was able to move so many people so quickly, the subway became the ultimate urban density amplifier, allowing the apartment buildings and office towers of Manhattan to be built side-by-side, and turning a 26-square-mile island of gneiss, marble, and schist into one of the world's greatest metropolises, where millions could live and trade services, goods, and ideas swiftly and efficiently.

Given how badly it was neglected in the twentieth century, it's a miracle that New York's subway survived into the twenty-first at all. In the mid-1950s, when the underfunded system's rolling stock was already forty years old, the city adopted an official policy of deferred maintenance, kicking off a long decline that tracked the city's sagging fortunes. The nadir came in the early '80s, as motors fell from brackets and trains burst into flames with depressing regularity. In one of the worst accidents, an antique signal failed, causing a Manhattan-bound local to slam into the back of a train waiting in a Brooklyn tunnel, killing the motorman and injuring 135 riders. When author Paul Theroux spent a week riding the rails in 1981, he discovered a Dickensian underworld of loopers (car-hopping purse-snatchers), skells (vagrants), shoeflies (undercover transit cops), and lushworkers (drunk-rolling pickpockets), where transit workers were burned alive in token booths for kicks.

"What is amazing," Theroux concluded, "is that back in 1904 a group of businessmen solved New York's transport problems for centuries to come. What an engineering marvel they eventually created in this underground railway! And how amazed they would be to see what it has become, how foul-seeming to the public mind." It was as though Theroux had stumbled upon a rusty musée mécanique in the jungle, kept running, barely, by the local tribespeople.

Something had to change, and it did. Shortly after Theroux's visit, the Transit Authority declared war on graffiti, hauling in cars nightly and scrubbing them clean of the day's accumulated Wildstyle tags. (Spray paint is now a thing of the past: the modern vandal has resorted to etching the windows with acid.) Antique and defective trains were replaced by Canadian-made cars that now average 690,000 miles between breakdowns—one hundred times the '80s norm. In the last thirty years, $75 billion has been poured into the system.

Not that grit has entirely disappeared. Among the subways of the world, New York's is a utilitarian system. With a few exceptions, the stationsare shallow: on the avenues, trains can be heard clattering through the sidewalk grates, raging uptown and down only a few yards beneath the feet of pedestrians. "There's the smelly essence of New York down there," intoned Manhattan's pop-poet laureate Lou Reed in the days of subway vigilante Bernhard Goetz, and the underground has retained its velvety stench of soot and sweat, mixed with the roasted nut odor of dust from overheated brake shoes settling on the puddles of rainwater between the tracks. On the concourses, freckled black with the chewing gum of the ages, pillars cut off sightlines, the ceilings seem to be only inches overhead, and the dry heat and jaundiced light enfold you, forcing you into a new, almost theatrical relationship with the city.

For anybody who grew up with stories of a system going to hell beneath a crumbling metropolis, it is remarkable just how well the New York subway works. Most trains are air-conditioned, and on the Grand Central platforms, giant overhead air-cooling units create an oasis that, on hot days, actually tempts you to linger underground. (I've spent a couple of Julys in New York. That people here rode the subways for most of the twentieth century without air-conditioning is a testament to their fortitude. That Parisians and Londoners are still asked to do without it should be considered cruel and unusual punishment.) These days, almost everybody takes the train; depending on the traffic, it can be faster than hailing a cab. Even billionaire mayor Michael Bloomberg sometimes rides the express to City Hall on the Lexington Avenue line—perhaps as often as twice a week.1 Subway ridership, which bottomed out in 1977 at a billion rides a year, is once again approaching the record highs of the post-war years, when two billion trips were taken on the elevateds and subways.

Most important, New York is finally building more transit infrastructure. In addition to the 7-line extension, the East Side Access tunnels, which are being drilled beneath the Waldorf-Astoria Hotel and the Seagram Building at a cost of $7.2 billion, will allow Long Island Rail Road commuters to arrive at Grand Central Terminal, saving a half hour of backtracking to East Side offices from the usual terminus at Penn Station. And the warren of corridors and stairs at Fulton Street in Lower Manhattan, which local transit campaigners have wryly dubbed a "funhouse that'sno fun," is finally being rationalized into an easy-to-use east-to-west concourse. (The Access to the Region's Core Project, whose three tunnels between New Jersey and midtown Manhattan would have doubled the number of rush-hour commuter trains serving Penn Station, was canceled by New Jersey's governor in 2010. Since then, the Bloomberg administration has reportedly been at work on implementing a further extension of the number 7 line across the Hudson River to New Jersey, a plan that would make the line significantly more cost effective.)

But the most eagerly awaited project of them all—one in the offing since the fare was a nickel and the Brooklyn "Trolley" Dodgers were hitting homers at Ebbets Field—has got to be the Second Avenue subway. When the widely hated elevated tracks on Second Avenue were torn down for scrap iron during the Second World War, and the Third Avenue El was demolished fifteen years later, it was with the understanding that a subway would soon replace them. Despite a number of promising starts, the Second Avenue subway never got built. The Lexington Avenue line, meanwhile, has had to do all the heavy lifting on the east side: its number 4, 5, and 6 trains carry 1.7 million people a day—equal to the ridership of Boston's "T," Chicago's "L," and Washington's Metro combined—making it the busiest transit line on the continent. Estimated price tag for the Second Avenue Subway, when it is completed, maybe, in 2016: seventeen billion dollars.

Does Manhattan really need this much subway? Definitely. In a week riding the Lexington Avenue line during the rush hour, I never got a seat. In fact, there wasn't a morning I rode when the trains weren't at crush load, which means 160 people were crammed into cars built for 110. Even with a train arriving every two minutes, commuters stand five deep on the main platforms. The infrastructure, meanwhile, is showing its age. At Union Square, the curved tracks make arriving trains squeal (at 98.6 decibels, the New York Post discovered, enough to cause hearing damage) and result in an 18-inch gap between train and platform at many doors. To solve the problem, early Industrial Age technology was mobilized: moving metal flanges snap into place from the edge of the platform when the train arrives. There is no way to decrease the headway—the amount of time—between trains; thanks to analog relays and signals that date from the 1930s—and look, as one subway commentator told me, "like the switchboard at the Grand Hotel"—trains are already running as close together as is safely possible. New York's subway may be running better than it has been in decades, but compared to many European and Asian systems, it is still in shockingly bad shape.

The rationale for the Second Avenue subway has always been a simple one: scooping up commuters on the far East Side will significantly reduce the atrocious crowding on the overtaxed Lexington. So far, however, work has been delayed by, successively, the Depression, the Second World War, the Korean War, and the fiscal meltdown of the '70s. This time around, work has actually begun beneath the streets of the Upper East Side, and the sandhogs have lowered a tunnel-boring machine into the launch box at 96th Street. Riding the "T," as the new line will be called, will likely be a disorienting experience for veteran straphangers: the bright, well-lit stations will have column-free mezzanines and dizzyingly high ceilings—more like Washington's Metro than the claustrophobic stations New Yorkers are used to.

That is, if this apparently cursed subway ever sees the light of day. Given the state of civic finances, its future is by no means guaranteed. With the real estate taxes that are the source of much of the MTA's funding in serious decline, the authority announced in 2010 it was facing a $900 million budget shortfall, and would have to cut basic service on three subway lines. Which raises the question: If the MTA can't even keep existing lines running, how can it afford to build a new one?

In a city this compact, populous, and wealthy, sustained investment in transit should be a no-brainer. Only 5 percent of daily commuters to Manhattan's central business district arrive by car; the rest get to work by foot, bicycle, or on some form of transit. The subway is the sine qua non of Manhattan; it keeps the economy of the city, the state, and the entire Northeast thrumming. Shut it down, even for a day, and New York City turns into Podunk.

When it comes to transit, though, some people just can't seem to do the math. It never pays its own way, goes the refrain; or The crooks that run the system are making money hand over fist; or the classic We should take all that money and use it to build more roads. It is the kind of reasoning that has turned the Second Avenue subway, a simple replacement project that should have been completed half a century ago, into the line that time forgot.

As a brief look at the history of public transport in New York shows, it has ever been thus.

Copyright © 2012 by Taras Grescoe. Reprinted by permission of Times Books, an imprint of Henry Holt & Company.