It happened on October 27th: New York City subway opens

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The first underground line of the New York City subway opened on October 27th, 1904 -- 35 years after the first elevated line.

The construction of an underground transit system had proven necessary after New York City, which consisted of Manhattan and the Bronx, merged with Queens, Staten Island, and Brooklyn. Today, the subway network has 24 lines and over 5 million daily passengers.

Source: Wikipedia