However, tunnelling shields were required for deeper sections, such as the Harlem and East River tunnels, which used cast-iron tubes.
(In 1912, workers excavating for the present-day BMT Broadway Line dug into the old Beach tunnel; today, no part of this line remains as the tunnel was completely within the limits of the present day City Hall Station under Broadway.
By the time the first subway opened in 1904, the lines had been consolidated into two privately owned systems, the Brooklyn Rapid Transit Company (BRT, later Brooklyn–Manhattan Transit Corporation, BMT) and the Interborough Rapid Transit Company (IRT).
By annual ridership, the New York City Subway is the busiest rapid transit rail system in both the Western Hemisphere and the Western world, as well as the seventh busiest rapid transit rail system in the world; only the metro (subway) systems in Beijing, Shanghai, Seoul, Tokyo, Guangzhou, and Moscow record higher annual ridership.
In 2015, the subway delivered over 1.76 billion rides, averaging approximately 5.7 million daily rides on weekdays and a combined 5.9 million rides each weekend (3.3 million on Saturdays; 2.7 million on Sundays).
The city built most of the lines and leased them to the companies.