The method of undermining the tracks on Broadway from 60th to 104th Streets was entirely different, for the conditions were not the same.  The street is a wide one with a 22-foot parkway in the center, an electric conduit railway on either side, and outside each track a wide roadway.  The subway excavation extended about 10 feet outside each track, leaving between it and the curb ample room for vehicles.  The construction problem, therefore, was to care for the car tracks with a minimum interference with the excavation.  This was accomplished by temporary bridges for each track, each bridge consisting of a pair of timber trusses about 55 feet long, braced together overhead high enough to let a car pass below the bracing.  These trusses were set up on crib-work supports at each end, and the track hung from the lower chords. (See photograph on page 42.) The excavation then proceeded until the trench was finished and posts could be put into place between its bottom and the track.  When the track was securely supported in this way, the trusses were lifted on flat cars and moved ahead 50 feet.

At 66th Street station the subway roof was about 2 feet from the electric railway yokes and structures of the street surface line.  In order to build at this point it was necessary to remove two large gas mains, one 30 inches and the other 36 inches in diameter, and substitute for them, in troughs built between the roof beams of the subway, five smaller gas mains, each 24 inches in diameter.  This was done without interrupting the use of the mains.

[Illustration:  MOVING BRICK AND CONCRETE RETAINING WALL TO MAKE ROOM FOR THIRD TRACK—­BROADWAY AND 134TH STREET]

At the station on 42d Street, between Park and Madison Avenues, where there are five subway tracks, and along 42d Street to Broadway, a special method of construction was employed which was not followed elsewhere.  The excavation here was about 35 feet deep and extended 10 to 15 feet into rock.  A trench 30 feet wide was first sunk on the south side of the street and the subway built in it for a width of two tracks.  Then, at intervals of 50 feet, tunnels were driven toward the north side of the street.  Their tops were about 4 feet above the roof of the subway and their bottoms were on the roof.  When they had been driven just beyond the line of the fourth track, their ends were connected by a tunnel parallel with the axis of the subway.  The rock in the bottom of all these tunnels was then excavated to its final depth.  In the small tunnel parallel with the subway axis, a bed of concrete was placed and the third row of steel columns was erected ready to carry the steel and concrete roof.  When this work was completed, the earth between the traverse tunnels was excavated, the material above being supported on poling boards and struts.  The roof of the subway was then extended sidewise over the rock below from the second to the third row of columns, and it was not until the roof was finished that the rock beneath was excavated.  In this way the subway was finished for a width of four tracks.  For the fifth track the earth was removed by tunneling to the limits of the subway, and then the rock below was blasted out.