Subway Through the Ages
The subway is an underground railway that usually serves as a part of a rapid transit system in an urban area. The New York subway is the largest subway in the United States.
Construction of the Subway
There are three methods to construction: cover and cut, cut and cover, and a third method involving a machine. The first method, cover and cut, is when you put a pre-laid slab of concrete on the ground, then dig under it. The second method, cut and cover, is when you dig the tunnel and then put the pre-laid slab over it. The third method, the one involving a machine, is where a machine is designed to drill a tunnel and line the hole at the same time. It does this by using a cutting head to drill the hole, and with other arms it immediately puts down the lining, (pre-laid concrete pieces).