On Fourth Avenue, north of Union Square to 33d Street, there were two electric conduit railway tracks in the center of the roadway and a horse car track near each curb part of the distance. The two electric car tracks were used for traffic which could not be interrupted, although the horse car tracks could be removed without inconvenience. These conditions rendered it impracticable to disturb the center of the roadway, while permitting excavation near the curb. Well-timbered shafts about 8 x 10 feet, in plan, were sunk along one curb line and tunnels driven from them toward the other side of the street, stopping about 3-1/2 feet beyond its center line. A bed of concrete was laid on the bottom of each tunnel, and, when it had set, a heavy vertical trestle was built on it. In this way trestles were built half across the street, strong enough to carry all the street cars and traffic on that half of the roadway. Cableways to handle the dirt were erected near the curb line, spanning a number of these trestles, and then the earth between them was excavated from the curb to within a few feet of the nearest electric car track. The horse car tracks were removed. Between the electric tracks a trench was dug until its bottom was level with the tops of the trestles, about three feet below the surface as a rule. A pair of heavy steel beams was then laid in this trench on the trestles. Between these beams and the curb line a second pair of beams were placed. In this way the equivalent of a bridge was put up, the trestles acting as piers and the beams as girders. The central portion of the roadway was then undermined and supported by timbering suspended from the steel beams. The various gas and water pipes were hung from timbers at the surface of the ground. About four sections, or 150 feet, of the subway were built at a time in this manner. When the work was completed along one side of the street it was repeated in the same manner on the other side. This method of construction was subsequently modified so as to permit work on both sides of the street simultaneously. The manner in which the central part of the roadway was supported remained the same and all of the traffic was diverted to this strip. [Illustration: SUPPORT OF ELEVATED RAILWAY STATION AT 42D STREET AND SIXTH AVENUE]