Several stories beneath the streets of the 17th arrondissement, a tunnel-boring machine 25 feet in diameter is grinding through the wet Parisian earth. After a few hours of gains, engineers pause the drilling long enough for the machine to lock together the curved trapezoids of concrete that form the tunnel wall. Dig, build, repeat. The cycle continues through the night, every night, with the whole sunken work site proceeding south 40 feet a day toward the Gare Saint-Lazare, Paris’s second-busiest rail station. Seventy men on the world’s slowest train to Paris.

Here begins the most ambitious new subway project in the Western world. The extension of Line 14 is but the first leg of the Grand Paris Express, a $25 billion expansion of the century-old Paris Métro. By the time the project is completed in 2030, the system will have gained four lines, 68 stations, and more than 120 miles of track. Planners estimate that the build-out will boost the entire network’s ridership by almost 40 percent.

Top left: A tunnel-boring machine. Bottom left: Workers constructing the tunnel wall. Right: A construction site in Paris’s expanding Métro system. (RATP-ING-Isabelle Bonnet)

The goals: Reduce the smog-choked region’s car traffic. Link business districts, airports, and universities. Ease social ills by knitting together the French capital’s isolated and troubled banlieues, much as the initial Métro construction did for the outlying districts of Paris proper at the dawn of the 20th century.

Once the drill now inching its way underneath the 17th arrondissement has reached the existing Line 14 terminus, it will reverse course and head—like all future subway construction in Paris—back toward the suburbs. Those suburbs don’t look much like their American equivalents. Europe’s largest business district (La Défense) lies outside Paris, as do the world’s largest fresh-produce market, a handful of universities, most of the region’s public housing, and several small cities with population densities higher than that of Paris itself. Not even one in five of the region’s residents live inside the French capital’s boundaries—a lower ratio of core population to suburban population than in London, Madrid, Barcelona, Berlin, Hamburg, Milan, or Rome.

A rendering of one of the expanded Métro stations (Kengo Kuma and Associates)

The region’s transportation system hasn’t caught up to this reality. The founding engineer of the Métro, Fulgence Bienvenüe, is said to have endeavored to place a station within 400 meters of every point in Paris—a goal nearly realized within the city limits. But beyond the Boulevard Périphérique, the ring road that bounds Paris, the tracks of the Métro and its long-distance partner, the RER commuter rail, protrude like spokes from a hub. Train travel between neighboring Parisian suburbs often requires a long and inefficient journey into and then back out of Paris. To a straphanger, suburban Paris is a series of islands linked to the Parisian mainland but not to one another.

A map of the expanded Métro system (Apur; Société du Grand Paris)

Three of the new lines will run north and east of Paris, through Seine-Saint-Denis, the poorest of the 96 departments in France. Among French cities with at least 50,000 people, six of the seven with the highest percentage of foreign-born residents are in Seine-Saint-Denis. Residents of Clichy-sous-Bois, where the riots that swept the region in 2005 began, will for the first time find central Paris within a 45-minute train ride. The town of Saint-Denis, the site of the standoff between police and the terrorists who struck Paris in November, will be home to the project’s largest train station. Designed by the Japanese architect Kengo Kuma, the junction is expected to handle 250,000 passengers a day.

Top: The city of Clichy-sous-Bois, where the 2005 riots began. Bottom left: Original Métro construction. Bottom right: A French police officer. (Joel Robine/AFP/Getty; Wikimedia; Michel Spingler/AP)

Benoît Quessard, an urban planner for the local government, told me that he sees the expansion as not merely “an economic wager but also a social one.” In this sense, it will test an old Parisian belief about the Métro conferring, beyond convenience, a kind of citizenship on its riders. In 1904, four years after the first line opened, the writer Jules Romains predicted that the system would be a “living, fluid cement that will succeed in holding men together.”