The most crowded train in the NYC Subway system is often considered to be the 4, 5, and 6 trains during peak hours. These lines, collectively known as the Lexington Avenue Line, serve a significant portion of Manhattan and the Bronx, and experience high ridership due to several reasons: 1.

The most used and busiest subway line in the New York City MTA system is the IRT Lexington Avenue Line, which carries the 4, 5, and 6 trains. Serving the East Side of Manhattan, this line is a vital artery for the city's commuters, often handling over 1.3 million riders on an average weekday—a figure that exceeds the total daily ridership of many entire transit systems in other major U.S. cities. The line runs from the Bronx down to Lower Manhattan and Brooklyn, passing through major hubs like Grand Central-42nd Street and Union Square. Because it is the only major line serving the densely populated East Side (until the full completion of the Second Avenue Subway), it is frequently overcrowded during rush hours. In 2026, the MTA has implemented more "Countdown Clocks" and increased service frequency on the 6 train to manage this massive volume, but it remains the undisputed "heavy lifter" of the New York City transit landscape.