Few cities in the world are as closely linked to their subways as New York City — the vast network helped shape the city and now carries nearly six million people a day. So when the most ambitious expansion of the subway system in half a century opens on Sunday, it will be a transformative moment, promising to alter the future of a large slice of Manhattan.

The new Second Avenue subway will provide badly needed relief to one of New York’s most congested transit corridors and is expected to be a boon to the local economy, making restaurants and stores suddenly easier to reach. But even as the city celebrates a line many doubted would ever open, its arrival has prompted fears that rising rents could force out longtime residents and shops — the kind of displacement that has swept through many other parts of an increasingly affluent New York and deepened its inequality.

People living near three new stations at 72nd, 86th and 96th Streets could face rent increases as high as $462 per month, according to a report by StreetEasy, a real estate website. Sleek high-rises are already popping up above the walk-up apartment buildings that have served as first homes for many New Yorkers.

One of those is Dina Zingaro, who gravitated to the neighborhood when she moved from New Jersey. She and a roommate pay $2,400 a month for a two-bedroom apartment on the fifth floor of a walk-up building.