The Civil War put a temporary dent in New York ’s economy, but by the 1880s, it was back in full force. Corporations all over the state doubled, tripled, or even quadrupled in size, with a corresponding explosion of activity in commerce, transportation, banking, and, especially, manufacturing fields.
New York City  was in its full glory. In 1892, 1,265 millionaires lived either in the city or its suburbs. In 1895, the city housed nearly 300 companies worth over one million dollars—more than the next six largest cities combined. In 1898, New York annexed Brooklyn , Queens , Staten Island , and the Bronx , thereby increasing its area from 23 to 301 square miles.
The rich and the powerful flocked to New York City from all over the country, and the social elite were soon defined as the “Four Hundred”—the maximum number of guests who could squeeze into Mrs. Astor’s 5th Avenue ballroom. Investment bankers such as J. P. Morgan and August Belmont became household names, as did leaders of commerce and industry such as John D. Rockefeller, Andrew Carnegie, and F. W. Woolworth.
New York City became the nation’s cultural capital as well. Theaters sprang up along Broadway , and the Metropolitan Museum of Art  and the Metropolitan Opera  opened their doors in 1880 and 1890, respectively. Walt Whitman sang the city’s praises in poems such as “Leaves of Grass” and “Crossing Brooklyn Ferry,” and Henry James and Edith Wharton reported on the lives of the upper crust in Washington Square and The Age of Innocence.
But the years surrounding the turn of the century also had a darker side. Between 1880 and 1919, a new wave of more than 17 million immigrants—this time mainly from Southern and Eastern Europe—swept into New York City. Many settled in the Lower East Side, where they worked miserable, low-paying jobs in the garment industry. Overcrowding became a serious problem; by 1900, more than two-thirds of the city’s residents were crowded into some 80,000 tenements in Manhattan  and Brooklyn . The Lower East Side  had a population density of 209,000 people per square mile, equal to that of today’s Bombay.
Immigrants who settled upstate to work the region’s many burgeoning factories suffered as well. Often illiterate and unable to speak fluent English, they were subject to exploitation, poor health and housing conditions, pollution, and increasing crime.
The larger the cities grew, the greater became the need for improved transportation. Horsecars gave way to streetcars, and by 1900, nearly every city upstate had a streetcar system.
In 1904, Manhattan  opened its first subway, long after London (1863) and shortly after Boston  (1897). But New York’s subway system would soon be distinguished for both its enormous size and its technological innovations. Within a year after opening, New York’s subway—then just a single line running up Park Avenue, across 42nd Street, and up Broadway—was carrying over 600,000 passengers per day. By 1937, the city boasted over 700 miles of track handling 4.2 million passengers per day. Today, the subway system still has about 700 miles of track, but it handles only about 3.5 million passengers a day.