The opening of the first segment of the line — an extension of the Q train to 96th Street — promises to lighten the crush of passengers on the Nos. 4, 5 and 6 trains along Lexington Avenue, the nation’s most overcrowded subway line, which had been the only line on the Upper East Side of Manhattan. When the stations opened shortly before noon, they were quickly filled with giddy riders both young and old, and strollers, suitcases and dogs — all familiar sights across the system.

But for all the excitement, the line, with just three new stops, is much more modest than the ambitious route running the length of Manhattan that was once envisioned. It serves a relatively affluent and not very diverse part of the city, which has more than eight and a half million people and many low-income and minority residents who live far from a subway line. With the subway reaching its highest ridership levels since 1948, much of the aging system is plagued by crowding and delays, even as subway and bus fares are expected to rise again in March.

Still, there was reason to cheer. 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 and bridges to its public transit systems. Few new subway stations have opened in recent years, even as expansive subway networks have sprouted in Asia, and most American cities never built any in the first place.