_Cross-Section._--The subway being built for two tracks in some places and for four tracks in other places, it was necessary to vary the form and dimensions of the cross-section. The cross-sections actually adopted are of three types. Fig. 115 shows the section known as the wide-arch type, in which the lining is solid masonry. The second type was known as the double-barrel section, and is shown by Fig. 116. The third type of section is shown by Fig. 117. The lining consists of steel columns carrying transverse roof girders, the roof girders being filled between with arches, and the wall columns having concrete walls between them. The wide-arch type and the double-barrel type of sections were employed in some portions of the Tremont St. line, where the traffic was very dense, since it was possible to construct them without opening the street. Much of the wide-arch line was constructed by the use of the roof shield, which is described in the succeeding chapter on the shield system of tunneling. [Illustration: FIG. 116.--Double-Barrel Section, Boston Subway.] _Methods of Construction._--Several different methods were employed in constructing the subway. Where ample space was available, the single wide trench method of cut-and-cover construction was employed, the earth being removed as fast as excavated. In the streets, except where regular tunneling was resorted to, the parallel trench or transverse trench cut-and-cover methods were employed. In the transverse trench method, trenches about 12 ft. wide were excavated across the street, their length being equal to the extreme transverse width of the tunnel lining, and their depth being equal to the depth of the tunnel floor. These trenches were begun during the night, and immediately roofed over with a timber platform flush with the street surface. Under these platforms the excavation was completed and the lining built. As each trench or “slice” was completed, the street above it was restored and the platform reconstructed over the succeeding trench or slice. During the construction of each slice the street traffic, including the street cars, was carried by the timber platform. [Illustration: FIG. 117.--Four-Track Rectangular Section, Boston Subway.] [Illustration: FIG. 118.--Section Showing Slice Method of Construction, Boston Subway.]