how is it constructed?
The typical tunnel construction method was cut-and-cover
New York City Subway
hen the IRT subway debuted in 1904, the typical tunnel construction method was cut-and-cover. The street was torn up to dig the tunnel below before being rebuilt from above. Traffic on the street above would be interrupted due to the digging up of the street. Temporary steel and wooden bridges carried surface traffic above the construction.

Contractors in this type of construction faced many obstacles, both natural and man-made. They had to deal with rock formations and groundwater, which required pumps. Twelve miles of sewers, as well as water and gas mains, electric conduits, and steam pipes had to be rerouted. Street railways had to be torn up to allow the work. The foundations of tall buildings often ran near the subway construction, and in some cases needed underpinning to ensure stability.

This method worked well for digging soft dirt and gravel near the street surface. Tunnelling shields were required for deeper sections, such as the Harlem and East River tunnels, which used c
what do they call the subway in new york Rapid transit
wiki:19918500
New York City Subway [SEP] Construction methods
When the IRT subway debuted in 1904, the typical tunnel construction method was cut-and-cover. The street was torn up to dig the tunnel below before being rebuilt from above. Traffic on the street above would be interrupted due to the digging up of the street. Temporary steel and wooden bridges carried surface traffic above the construction. Contractors in this type of construction faced many obstacles, both natural and man-made. They had to deal with rock formations and groundwater, which required pumps. Twelve miles of sewers, as well as water and gas mains, electric conduits, and steam pipes had to be rerouted.