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.

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.

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.

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.”

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