In the parallel trench method, short parallel trenches were dug for the opposite side walls, and also for the intermediate columns, and completely roofed over during the night. Under this roofing the masonry of the side walls and column foundations and the columns themselves were erected. When the side walls and columns had been erected, the surface of the street between them was removed, the roof beams laid, and a platform covering erected, as shown by Fig. 118. This roofing work was also done at night. The subsequent work of building the roof arches, removing the remainder of the earth, and constructing the invert, was carried on underneath the platform covering which carried the street traffic in the meantime. The successive repetition of the processes described constructed the subway. Where the traffic was very dense on the street above, tunneling was resorted to. For small portions of this work the excavation was done in the ordinary way, using timber strutting, but much the greater portion of the tunnel work was performed by means of a roof shield. In the latter case, the side walls were first built in small bottom side drifts and were fitted with tracks on top to carry the roof shield. The construction and operation of this shield are described fully in the succeeding chapter on the shield system of tunneling. _Masonry._--The masonry of the inclined approaches to the subway consists simply of two parallel stone masonry retaining walls. In the wide-arch and double-barrel tunnel sections, the side walls are of concrete and the roof arches are of brick masonry. In the other parts of the subway the masonry consists of brick jack arches sprung between the roof beams and covered with concrete, of concrete walls embedding the side columns, and of the concrete invert and foundations for the columns. Figs. 115 to 118 inclusive show the general details of the masonry work for each of the three sections. The inside of the lining masonry is painted throughout with white paint. _Stations._--The design and construction of the stations for the Boston Subway were made the subjects of considerable thought. All the stations consist of two island platforms of artificial stone having stairways leading to the street above. The platforms are made 1 ft. higher than the rails. The station structure itself is built of steel columns and roof beams with brick roof arches and concrete side walls. Its interior is lined with white enameled tiles. The intermediate columns are cased with wood, and have circular wooden seats at their bottoms. Each stairway is covered by a light housing, consisting of a steel framework with a copper covering and an interior wood and tile finish.