NEW YORK — The Second Avenue subway opened in New York on Sunday, with thousands of riders flooding into its polished stations to witness a piece of history nearly a century in the making.

They descended beneath the streets of the upper East Side of Manhattan to board Q trains bound for Coney Island in Brooklyn. They cheered. They wept. They snapped selfies with colorful mosaics lining the stations.

The first day of 2017 felt like a new day for a city that for so long struggled to build a subway line that has been sorely needed. In a rare display of unbridled optimism from hardened New Yorkers, they arrived with huge grins and wide eyes taking in the bells and whistles at three new stations.

It was an extraordinary moment for New York City, for its sprawling subway system, and for the upper East Side after decades of failed efforts to bring the subway to one of the few corners of Manhattan it did not reach.

The new line, which is an extension of the Q train to 96th Street, promises to lighten the crush of passengers on the No. 4, 5, and 6 trains along Lexington Avenue, the nation’s most overcrowded subway line.

At 72nd Street, subway riders popped champagne. On board, their eyes flickered as the train zoomed through a new tunnel that was dug deep below Manhattan. They crossed the platform and boarded a returning train to do it all again.

The opening of a new subway line is a rare occasion in the United States and comes at a time of mounting concern about the deteriorating state of the nation’s infrastructure, from its roadways to its bridges to its public transit systems.

Few new subways have opened in decades and most cities never built them in the first place, even as sprawling subway networks have sprouted in Asia.

So the arrival of the long-delayed Second Avenue subway, which was first proposed in the 1920s, was a notable achievement for the often-vilified Metropolitan Transportation Authority, which runs the city’s network of subways, buses, and commuter railroads. The first phase of the project took nearly a decade to build and cost $4.4 billion.

With the opening, the city’s subway added three new stations to the map, bringing the total number of stations to 472.

New York Times