Fig. 149 shows cross-section of a plate-girder over-line bridge, 32 feet span, carrying a private road, 12 feet wide, over a double-line railway. The road traffic being small, the floorway was constructed of creosoted planking carried on rolled I-iron cross-girders placed at 3 feet 8 inch centres, and riveted to the main girders. The horse-tread track was provided with a second layer of planking, laid transversely, to take up the wear, cross battens, 4 inches by 2 inches, being placed at 12-inch centres, and sand spread between to give good foothold. A light lattice-work parapet was bolted on to the top of the main girders. Fig. 150 gives cross-section of a plate-girder over-line bridge, 30 feet span, carrying a private road, 20 feet wide, over a double-line railway. The main girders are tied together by lattice-work bracing, spaced at 7-foot centres. Curved wrought-iron plates are laid across from girder to girder, and butt against a narrow horizontal plate, which forms part of the upper boom. The curved plates are riveted on to the top of girder, and form a continuous iron floor, or deck, from side to side of the bridge. Upon this iron floor is laid an ordinary asphalte roadway. The outside girders are made deeper, and carry an ornamental cast-iron parapet. In some bridges of a similar construction, the roadway is formed of creosoted wooden block paving, on a foundation of asphalte. [Illustration: BRIDGE CARRYING THE D. W. AND W. RAILWAY (LOOP LINE) OVER AMIENS STREET, DUBLIN. [_To face p. 144._] [Illustration: Fig. 146, 147, 158, 159, 148] Fig. 151 shows cross-section of a plate-girder over-line bridge, 28 feet span, carrying a public road, 35 feet wide, over a double-line railway. The main girders, 2 feet 4 inches deep, are placed at 5 feet 2 inch centres, and are tied together by plate-iron cross-bracing 2 feet deep. Jack-arches of brickwork, 9 inches thick, are built in between the main girders, the haunching being filled in with concrete. The entire surface is covered over and made watertight with asphalte, on which is laid the metalling of the roadway. The outside girders are made considerably deeper, and have strong cast-iron-plate parapets bolted on to the top booms. There is no doubt that jack-arching of brickwork or concrete makes a very strong and permanent floorway, but its dead weight is very great, and its adoption is not to be recommended 147 where iron or steel plate troughing can be obtained at a moderate price. Fig. 152 gives cross-section of plate-girder over-line bridge, 41 feet 6 inch span, carrying a public road, 25 feet wide, over three lines of way. Two main girders are used, of sufficient depth to form parapets or screens for the finished roadway. Plate cross-girders, placed at 6 feet 6 inch centres, are riveted to the web-plate and lower angle irons of main girders; and on these is placed a flooring of plate-iron longitudinal troughing to carry the metalled roadway.