total distance of about 5 miles. The three-track viaducts are carried
on two column bents where the rail is not more than 29 feet above the
ground level, and on four-column towers for higher structures. In the
latter case, the posts of a tower are 29 feet apart transversely and
20 or 25 feet longitudinally, as a rule, and the towers are from 70 to
90 feet apart on centers. The tops of the towers have X-bracing and
the connecting spans have two panels of intermediate vertical sway
bracing between the three pairs of longitudinal girders. In the low
viaducts, where there are no towers, every fourth panel has zigzag
lateral bracing in the two panels between the pairs of longitudinal
girders.

[Illustration: PROFILE OF HARLEM RIVER TUNNEL AND APPROACHES]

[Illustration: SECTION OF HARLEM RIVER TUNNEL DURING CONSTRUCTION]

[Illustration: ASSEMBLING IRON WORK ON PONTOON--HARLEM RIVER TUNNEL]

The towers have columns consisting as a rule of a 16 x 7/16-inch web
plate and four 6 x 4 x 5/8-inch bulb angles. The horizontal struts in
their cross-bracing are made of four 4 x 3-inch angles, latticed to
form an I-shaped cross-section. The X-bracing consists of single 5 x
3-1/2-inch angles. The tops of the columns have horizontal cap angles
on which are riveted the lower flanges of the transverse girders; the
end angles of the girder and the top of the column are also connected
by a riveted splice plate. The six longitudinal girders are
web-riveted to the transverse girders. The outside longitudinal girder
on each side of the viaduct has the same depth across the tower as in
the connecting span, but the four intermediate lines are not so deep
across the towers. In the single trestle bents the columns are the