When the 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 for digging soft dirt and gravel near the street surface.
However, mining shields were required for deeper sections, all of which used 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.