LONDON — Up an alley, beyond some hoarding, through what can feel like Harry Potter’s secret portal, the underworld of an unfinished Crossrail station sprawls beneath the traffic and commotion of Tottenham Court Road. Escalator banks descend through a sleek, silent black ticket hall where towering, empty, white-tiled passageways snake toward the new, vaulted train platform, curving like a half moon into the subterranean darkness.

Crossrail is not your average subway. London’s $20 billion high-speed train line, which plans to start taking passengers late next year, is Europe’s biggest infrastructure project.

It will be so fast that crucial travel times across the city should be cut by more than half. The length of two soccer pitches, with a capacity for 1,500 people, its trains will be able to carry twice the number of passengers as an ordinary London subway. While Londoners love to moan about their public transit network, by comparison New York has barely managed to construct four subway stops in about a half-century and its aged, rapidly collapsing subway system now threatens to bring the city to a halt.

But standing one recent morning on that empty Crossrail platform, where construction workers in orange gear and hard hats hauled shiny metal panels to line the walls, I still couldn’t help wondering whether the new train leads toward another glorious era for this city, or signals the end of one.

Before Britain voted last summer to leave the European Union, Crossrail was conceived for a London open to the world and speeding into the future. Now, with Brexit, the nightmare scenario is that this massive project, to provide more trains moving more people more quickly through a growing city, ends up moving fewer people more quickly through a shrinking city.

Crossrail was built by a Britain whose strength grew, for better and worse, out of a longstanding, stodgy but reliable confidence that the country knew itself and where it hoped to go in the century ahead. It is no longer even certain that Prime Minister Theresa May will survive the year.

It was an especially unpromising sign this spring when Mrs. May’s Conservative government, as if fearing exactly what anti-Brexiters predicted about an economic downturn, issued a campaign manifesto that conspicuously omitted funding for Crossrail 2, the long-planned, $39 billion critical north-south sequel to Crossrail’s east-west line.

Since then, the government’s transport secretary has endorsed the project — provided that the city pay half the whopping cost, upfront. The semi-reversal suggested a grudging acknowledgment that, whatever the political fallout or economic prospects, Britain ultimately needs a thriving London all the more after Brexit.

During the past three decades, London has been transfigured by wild growth, much of it the consequence of government-sustained megaprojects: Along with Crossrail, there have been the stupendous renovations to King’s Cross and St. Pancras Stations, the wholesale invention of Canary Wharf, the addition of the Jubilee subway line, the Olympic makeover at Stratford in East London and the expansion of Heathrow Airport.

These megaprojects, in different ways, helped remake London into the great global city-state of Europe, a 21st-century melting pot and Sybaris of culture and free-market prosperity — at the same time that they clearly exacerbated underlying urban inefficiencies and stirred resentment elsewhere in England toward the city.

Crossrail was intended as a kind of democratizing corrective, at once shrinking the city and expanding on a vision of London as a great, inclusive metropolis. While it will whisk bankers at new speeds from their office towers and multimillion-dollar aeries to Heathrow, it will also help millions of now-marginalized, lower-income workers, unable to afford runaway home prices in and around the center of the city, to live in cheaper neighborhoods often far from their jobs.

But what if the flow of incoming bankers slows, if immigrants look elsewhere, if the excesses of European money and human capital that helped drive growth begin to dry up? As Brexit skeptics warned, the pound has lost value and inflation is starting to rise. Some companies are already making plans to move employees out of London.

And as London goes, so goes Britain.

A New Spine for an Old City

I spent a few days traveling the Crossrail route, trying to decipher what it might mean for London. In one respect, the train underscores and extends the city’s centuries-old, traditional identity as a sprawling, horizontal capital, an agglomeration of disparate, far-flung villages.

Extending roughly 70 miles, it is built to speed about 200 million passengers a year in a kind of Y from far to the west of the city, in the county of Berkshire, through Heathrow, to the heart of London, forking east to Shenfield in Essex and to the neighborhood called Abbey Wood, on the historically neglected southeast side of the Thames River. Linked with the existing Underground subway network, it will be rechristened the Elizabeth Line, inserting what is in effect a new steel-and-wheels spine into Britain’s capital.

“Crossrail is a culmination of years of serious thinking by experts and public officials about what London needs, the imbalance of east and west and how to unite the city,” said Ricky Burdett, an architect, city adviser and director of LSE Cities at the London School of Economics.

London, Mr. Burdett noted, is historically poor in the east, rich in the west and along the periphery, although that east-west distinction has eroded as gentrification has seeped outward. Underserved and long-disconnected East London neighborhoods like Shoreditch and Whitechapel in recent years have become chic and largely unaffordable to many Londoners. The city has added light-rail lines to help some of those areas, but only Crossrail is capable of “tying together many of the developments that have transformed London,” Mr. Burdett said.

No development on the west end of the Crossrail line is more ambitious than Heathrow. John Holland-Kaye, the airport’s chief executive officer, met me one morning in an empty conference room near the airport and instantly ticked off some figures.

Heathrow is not only Britain’s busiest airport. It is hoped that the airport’s expansion, based on the prospect of a new runway, might generate up to 180,000 new jobs across Britain, roughly 40,000 of them in London. Crossrail now promises to bring six million people and 80 percent of London’s corporations within an hour’s commute of the airport, up from three million and 50 percent today.

“When Crossrail is done, our employees could just as easily come from East London as from our local community,” Mr. Holland-Kaye said. “The train effectively opens up the whole of the city.”

Transit as Economic Engine

But Brexit threatens Heathrow’s economy with more restrictive customs and immigration rules. Mr. Holland-Kaye acknowledged the threat but framed Crossrail as a kind of hedge against Brexit. He cited as a precedent the area around King’s Cross, once notorious for drugs and prostitution, metamorphosed after the renovation of the decrepit King’s Cross station and its neighbor St. Pancras, now serving the Eurostar express train to Paris and other cities in Europe.

King’s Cross today is home to an art school, The Guardian, a cluster of high-tech medical research centers and Google’s future European headquarters. The point: Major, long-term infrastructure projects support game-changing investments. Crossrail proves Britain’s continuing commitment to this steel and concrete approach, Mr. Holland-Kaye said.

From Heathrow, riders will need just over a half-hour via Crossrail to travel east to Canary Wharf, the defunct docklands turned world financial hub, which today employs more than 112,000 people. When the site opened in the late 1980s, the aptly named Narrow Street was its only real access road. Canary Wharf went belly up. Then London broke ground for the Jubilee subway line, linking Canary Wharf by mass transit to the heart of the city, and international banks started moving in.

This is one reason George Iacobescu, Canary Wharf’s longtime chairman, helped lead the push for Crossrail. “London’s future prosperity depends on it,” he said. He summoned me into a big, bright white office where he stood behind a giant tabletop display of London (think Goldfinger’s model of Fort Knox). Mr. Iacobescu flicked switches on the table. One by one, they lit up various rail lines that today serve Canary Wharf, each line coinciding with increases in jobs and revenues. Theatrically, he paused before the last switch.

It illuminated Crossrail.

“We are home to many of the world’s great financial institutions,” Mr. Iacobescu said, pointing on the model to where Foster & Partners, the celebrated London-based architecture firm, has designed Canary Wharf’s Crossrail Station, a spectacular glass and timber tubular structure, docked like a giant cruise ship beside the firm’s HSBC tower. Nearby, Canary Wharf plans to build thousands of new luxury (and some affordable) homes and other developments by high-end architects like Herzog & de Meuron to turn Canary Wharf into more of a neighborhood. “We expect to create thousands more jobs,” Mr. Iacobescu said.

“The danger with Brexit,” he added, “is that if Britain gets out of the European Union and doesn’t keep the U.K. an attractive place for financial institutions, they will think twice about growing here. The issue isn’t banks leaving Canary Wharf. Most of them have long-term leases. The issue will be the pace of growth.”

But that’s not quite true. Because of Brexit worries, construction plans for several of Canary Wharf’s new buildings have already been put on hold. And long-term leases can always be broken.

“The bottom line is that nobody has the faintest idea yet what the Brexit effect will be,” Tony Travers, a veteran urban policy expert, told me. “Investments from the European Union may shrink. But why would investors from India or Canada or the United States be put off? If anything, Brexit may make Britain more likely to give them what they want.”

Lately, the Leadenhall Building, otherwise known as the Cheesegrater, the tallest tower in the old, central financial district called the City of London, designed by Rogers Stirk Harbour & Partners, sold to a Chinese property tycoon for $1.5 billion, the second-highest-ever sale of a building in Britain. Qataris were behind the Shard, the Renzo Piano-designed tower that is London’s sleekest skyscraper. Malaysians are developing the former Battersea Power Station, where Apple is an anchor tenant.

“The trajectory of real estate investment here is not only based on Europeans,” Mr. Burdett stressed.

So, Crossrail or Brexit?

Connectivity is destiny in the farthest reaches of East London. Thamesmead, a social housing development from the 1960s, lies a half-hour or so by bus beyond the train’s final stop and its ripple effects. Equivalent in area to all of central London, it is home to just 50,000 residents, once nearly all of them white and working-class, but today, increasingly, Nigerians. Peabody, a nonprofit housing organization, has announced plans to build hundreds more apartments in Thamesmead. Some lovely, neatly tended homes already exist alongside Brutalist blocks, rundown but now stylish (this is where “A Clockwork Orange” was filmed). That said, with the lowest average income in London, Thamesmead has few stores, little street life and an abundance of sewage treatment plants and prisons. It suffers from its isolation.

“Thamesmead was built on a promise of transit connectivity that never happened,” Teresa Pearce, a member of Parliament who represents the district, told me. “You see the results.”

By comparison, the final stop on that southeast spur of Crossrail is Abbey Wood, where Sainsbury’s, the chain store and a bellwether of gentrification and commercial investment, has lately opened a shop in anticipation of the train. Streets here are lined with terrace houses now occupied by plasterers and truck drivers. Record numbers of landlords in the area have been filing applications for renovations, believing that Crossrail will attract bankers and lawyers. Property values are expected to rise on average 10 percent around all future stations along the Crossrail route.

Change is even more acute one stop before Abbey Wood, in Woolwich. The Berkeley Group, a big British real estate company, is building 5,000 sleek, mostly high-end apartments around the future Crossrail station, which the developer paid millions to help construct.

Hugging the Thames River, Woolwich is the former site of the Royal Arsenal and Henry VIII’s dockyard, where Charles Darwin’s Beagle was built. Historically working-class, it, too, used to be all white but has come to attract Caribbean and Asian immigrants, with nearly 40 percent of residents today living in social housing. Berkeley’s development and the Woolwich Crossrail station are separated from the rest of Woolwich by a highway called Plumstead Road. On one side of the road, old Woolwich is a warren of modest shops and aged social housing.

On the other, baristas now dispense macchiatos on leafy patios. Signs advertise luxury apartments. A single, 31-year-old corporate lawyer employed in the City, Calum Docherty, who is hunting for a home, was intrigued by the Berkeley development. “But I’m still looking,” he told me. “Woolwich wasn’t anywhere on my radar before Crossrail. The train has expanded my concept of London.

“That said, it’s a bit of a mental jump to commit to borrowing half a million pounds,” he added. “With Brexit and all.”