Four 12 x 12-inch timbers were laid upon the surface. Standard
cast-iron yokes were placed upon the timbers at the usual distance
apart. Upon this structure the regular track and slot rails were
placed. The space between the rails was floored over. Wooden boxes
were temporarily laid for the electric cables. The usual hand holes
and other accessories were built and the road operated on this timber
roadbed. The removal of the tracks was made necessary because the rock
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.
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