As cities have become increasingly congested with vehicular traffic, mass transit systems have been developed to relieve traffic jams, taking passengers to their destinations without the need for automobiles. Subways serve as the nucleus of many transit systems and are generally underground rail lines that offer commuters a relatively speedy ride to and from urban centers. In most cases, the subway forms the nucleus of a larger network of ground-level and above-ground train and bus lines.

Two types of rail subways exist, the open-cut and the tubular. The open-cut subway is constructed by digging a trench, then covering it after the tunnel structure has been put into place. Open-cut subways are rectangular in shape.

The tubular subway, or tube, is constructed by boring a round linear hole parallel to the ground surface. The surface is penetrated only for passenger stations and ventilation shafts.

The first tunneling shield&mdash...