NYC Subway, Deep Underground Tunnel

Most of New York City's Subways were built in the early 20th century using the "cut-and-cover" method, which involved digging trenches, laying track and supports, and then cover over the exposed trench to make a tunnel. In a few places in the system, however, the subway was far enough underground that it had to be bored as a round tunnel.

In these deepest parts of the system, the tunnels are essentially round in cross-section, as opposed to the rectangular shape seen through most of the subway lines. Such round tunnels can be seen, for example, in all the tunnels where the subway crosses under rivers. The round shape not only facilitated the boring process, but it also allowed the tunnelers to create the tunnel as a series of reinforced ribs which help support the weight of the rock, water, and mud above.

The picture above shows a section of the subway where a line crosses underneath a river. The shape of the tunnel is essentially circular, with a flattened bottom on which the rails are laid. The doorway visible to the upper right in the picture leads to a parallel tunnel for trains going in the opposite direction.

Round tunnels such as this are the most dangerous type of subway or train tunnels because there is no clearance between the train and the walls of the tunnel. In such a narrow tunnel, the movement of the train-- which often travels very fast in the long stretches of river crossings-- also creates an incredibly forceful wind, functioning like the piston in a pump to drive air in front of it.