beneath them and the concrete around the yokes was so closely united
as to be practically monolithic, precluding the use of explosives.
Attempts to remove the rock from under the track demonstrated that it
could not be done without destroying the yokes of the surface railway.

[Illustration: SUPPORTING ELEVATED RAILROAD BY EXTENSION GIRDER--64TH
STREET AND BROADWAY]

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