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. This method worked well for digging soft dirt and gravel near the street surface. However, mining shields were required for deeper sections, such as the Harlem and East River tunnels, which used cast-iron tubes, segments between 33rd and 42nd streets under Park Avenue, 116th Street and 120th Street under Broadway, and 145th Street and Dyckman Street (Fort George) under Broadway and Saint Nicholas Avenue as well as the tunnel from 96th Street to Central Park North - 110th Street & Lenox Avenue, all of which used either rock or concrete-lined tunnels.
About 40% of the subway system runs on surface or elevated tracks, including steel or cast iron elevated structures, concrete viaducts, embankments, open cuts and surface routes. All of these construction methods are completely grade-separated from road and pedestrian crossings, and most crossings of two subway tracks are grade-separated with flying junctions.
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