[Illustration: FIG. 119.--Double-Track Section, New York Rapid Transit Railway.] The main line from the City Hall loop to about 102d St. consists of four tracks built side by side in one conduit, except for that portion under the present Fourth Ave. tunnel where two parallel double-track tunnels are employed. The West Side Line will consist of double tracks laid in one conduit, except across Manhattan St. and beyond 190th St., where it is carried on an elevated structure. The East Side Line consists of a double-track tunnel driven from 102d St., and the boulevard under Central Park to 110th St. and Lenox Ave., and two parallel circular tunnels excavated under the Harlem River,--the other portions of the road being double-track, subway and elevated structure. _Methods of Excavation._--Both the double-and four-track subway were built by using the different varieties of the cut-and-cover method. The single wide-trench method was used for the construction of the double-track line and also for the construction of the four-track line where the local conditions allowed it. The single narrow-trench method was used for the construction of the four-track subway at 42d St., to meet with the peculiar conditions of the traffic. Almost the total length of the four-track line of the subway was built by means of the two parallel side trenches. The slice method, so successfully employed in the Boston Subway, was used only on 42d St. west of 6th Avenue. _Lining._--The lining of the subway is of concrete, carried by a framework of steel. The floor consists of a foundation layer of concrete at least eight inches thick on good foundation, but thicker, according to conditions, where the foundation is bad. On top of this is placed another layer of concrete, with a layer of waterproofing between the two. In this top layer are set the stone pedestals for the steel columns, and the members making up the tracks. In the four-track subway, the steel framework consists of transverse bents of columns, and I-beams spaced about five feet apart along the tunnel. The three interior columns of each bent are built-up bulb-angle and plate columns of H-section. The wall columns are I-beams, as are also the roof beams; between the I-beams, wall columns, and roof beams there is a concrete filling, so that the roof of the subway will be made up of concrete arches resting on the flanges of the I-beams of the roof. The concrete used is of one part Portland cement, two parts sand, and four parts broken stones. The double-track subway is built in the same way, except that only one column is placed between the tracks for the support of the roof.