NEW YORK — Perhaps no space in the city is governed by as complex a set of social rules as the cramped subway car, that lurching human Petri dish that has spawned a thousand sociology papers. Its nuanced etiquette is known to generations of New Yorkers: head down, body clenched, contact of eyes or limbs a no-no.

But things are about to get interesting this week.

Four underground stations in Manhattan will come alive Tuesday with an amenity that has long eluded the city’s subterranean realm: cellphone service, loud and mostly clear. In a landmark trial, New York City Transit will debut a fiber-optic network that could be expanded to every underground platform by 2016.

The test amounts to a vast social experiment for millions of New Yorkers accustomed to the subway as a forced respite from the frenzy of modern life. The contemplative commute could yield to the on-the-go office. Books may be discarded for text messages and emails. And a convenient excuse to ignore a call may be rendered moot.

Most of all, the trial could lay bare the deep ambivalence of iPhone-addicted urbanites who lament their digital tethers, and yet guzzle so much smartphone data that the city’s cell networks often slow to a crawl.

For Johnny Rocco, a shoe designer commuting on the Q train last week, the idea of cellphones in the subways seemed a recipe for chaos. "Everyone will be on their phones on the platform," he said sadly, as he watched a middling guitarist in his subway car serenade riders for money. "It’s aggravating enough having to listen to that."

But asked if he might take advantage of the new service, Rocco smiled and shrugged. In reality, he said, he plans to be one of the worst offenders.

"You can’t not look into your phone," he said.

There are caveats — this is, after all, an attempt to bring 1990s-era technology to a transit system that has long been a citadel against the advances of the modern world. The phones are expected to work only on platforms, not in tunnels between stops, and only AT&T and T-Mobile subscribers will benefit. (Verizon has yet to agree to the trial.)

Cellphones, of course, have become integral to city life, be it checking directions or reporting a crime, and their appearance underground will put an end to those tense minutes when friends or children are out of reach.

"If anything, people may be letting trains go past because they have to stay online for a couple more minutes to get something done," said Anthony Townsend, a researcher in New York for the Institute for the Future, a Silicon Valley think tank. "I think we’ll see some odd and unusual behaviors like that."

Gene Russianoff, head of the Straphangers’ Campaign, praised the cellphone plan as a way to integrate the subway "into the daily realities of people’s lives." But he admitted that he would miss that immutable excuse to avoid emails or duck an unwanted call.

"You can build airplanes, but you lose the wonder of the sky," he said, evoking a speech from "Inherit the Wind," another saga of society’s struggles with change. "The fact that your boss can get a hold of you even more often than he or she normally does is probably not a welcome thing."

Other cities have enjoyed underground reception for years, and chattiness can be an issue. Riders in Tokyo were asked to refrain from taking calls at peak hours. In Boston, officials posted signs encouraging riders to speak at a reasonable volume. A recent expansion of cell service on the Washington Metro led to a spate of snatch-and-grab smartphone thefts.

If New York’s subway-goers get too loud, Edward I. Koch, the former mayor, suggested a simple reprimand: "Please, not so loud." And Russianoff noted that nearly 200 stations are above ground. "I’ve been dealing with cellphones on the F train for a decade," he said. "I don’t know how much it changes the balance of how people behave."

The Metro-North Railroad, notorious for its chatty passengers, plans to introduce cellphone-free quiet cars on some routes next month. So should we look forward to sitting in a quiet car on the A train anytime soon? A subway spokesman was blunt: "The answer is no."