In contrast to
light rail transit, heavy rail transit predominates in cities with
large population densities and demand for central business district
destinations or other ridership generators such as airports. These
rail systems are operating in eleven U.S. cities under fourteen
operating agencies. They are by no means static, as they are continuously
subject to further development and expansion to provide better service
to the public.
Today, twelve of our major airports are serviced by rail transit.
Over the years, since the first rail transit system was constructed
in New York City, rail development has been pursued to promote growth
and development in specific community areas, control growth patterns,
support system-wide accessibility goals, and meet environmental
and quality of life goals.
Companion to these goals, rail transit projects have been deployed
to relieve congestion or accommodate future growth in auto traffic.
Rail transit is but one part of the balanced transportation equation.
Operationally, these systems are characterized by their ability
to move large masses of people, at high rates of speed, free from
congestion of other vehicles occupying the same right-of-way. Hence,
they are built in subways under city streets or on elevated guideways
above the cityscape.
Concrete has become the most prevalent construction material facilitating
the heavy rail transit mode. Foundations, piers, guideways, and
stations are composed of the many construction options open to the
designers and builders of these systems. Concrete systems range
from cast-in-place and precast applications to prestressed and post-tensioned
elements for their principal guideways and structures.
Examples of ingenuity abound in the design of these systems. In
many, tracks are no longer laid on wood ties and ballast, but are
directly affixed to the concrete structure by means of fastening
devices that shield the system from noise and vibration. In subway
tunnels of some systems, the concrete trackway floats on rubber
pads to isolate ground-borne noise and vibration.
Station houses, platforms, maintenance facilities, and substations
may be constructed of cast-in-place or precast concrete elements
that enable the transit system to be strong, durable, and aesthetically
pleasing.