It's Monday-morning rush hour in Tokyo, and the city is hungry. Train stations devour people by the hundreds, swallowing them down escalators and onto platforms deep in their bowels. It's nearly impossible to reverse direction. Arms occasionally get broken. A train glides into the station, its doors open, and the crowd parts as hundreds more flow out. On the intercom, a melodic ditty speeds up its cadence, urging commuters to board. When the music stops, the doors close.

Inside, people are packed so tightly it's hard to breathe. Cars designed to hold about 160 cram in 300 to 400. Faces are splayed against backs; elbows dig into torsos. People clutch smartphones and contort themselves to check e-mail. No one speaks; they're standing too close.

"It's an intimacy befitting lovers, not commuters," says Chicago anthropologist Michael Fisch, whose book project, Between the Lines: An Anthropology of Love, Labor, and Death in Japan's Commuter Train Network, investigates the relationship between Tokyo's people and its transit. Part of an emerging subfield called the anthropology of infrastructure, his research explores how the train system itself shapes—and reflects—the rhythms of a digital city.

Carrying some 20 million people a day, the Japan rail network appears dangerously overtaxed, the remnant of a post–World War II demographic shift that brought record numbers into the city. "It's a system that's operating beyond capacity, a system that shouldn't work," says Fisch, a Japan expert who spent months riding trains between 2004 and 2008.

Yet it does work—with a precision that would boggle Chicago El riders. So accurate are the schedules, Fisch says, it's not uncommon for a rush-hour commuter to say she rides "the 7:43 a.m., third car, fourth door." For those who miss their train, another arrives in less than two minutes.

At the heart of the system is a balance between exactitude and uncertainty. With more than 12,000 trains running each day, arrivals and departures must be timed within seconds. Yet the crowds and the disturbingly regular rail suicides—an almost daily occurrence since the Japanese economy collapsed in the early 1990s—means unpredictability must be part of the transit equation.

To tackle these challenges, Tokyo's network uses a sophisticated model that breaks down the wall between man and machine, creating an environment where technology incorporates irregularity rather than trying to eliminate it, argues Fisch, an assistant professor of anthropology and social sciences. "There's no other system in the world that operates like this," he says of the Autonomous Decentralized Transport Operation Control System, implemented in 1996. Instead of one command station instructing trains to speed up or slow down, the network is partly decentralized, so the trains' computers can adjust if faced with the unexpected and refine an ever-changing traffic map.

"Basically, it's a smart network. The train itself makes decisions—and learns from those decisions," says Fisch. Each decision gets dispersed to the rest of the network so the system can operate smoothly, even during the frenzied rush hour. That dynamic, Fisch argues, produces a "radically new kind of urban space" where the environment reacts to city dwellers as they move through it.

Tokyo's train infrastructure has altered everyday life in other ways as well. Consider, Fisch says, the packed train cars where no one speaks, but riders click away on smartphones, chatting online through "commuter networks."

"Their bodies are being transported in one kind of network, but they're surfing another one," he says. The modern story of technology has focused on the division between the technological and the human, Fisch says. "Everybody is really connected—and yet disconnected." Ultimately, he says, it's a reality "where you really can't make divisions between the human and the technological."