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