never knows its exact path of travel until a short distance before reaching
a point of divergence.
Reduce-speed signals, however, are displayed
sufficiently in advance of a change of track to give an engineer time to
slow down. The engine crew relies on the permissive and restrictive aspects
of the signals, automatically displayed when the dispatcher opens or closes
switches ahead of them, and they know they are taking the route prescribed
by the dispatcher.
b. Safety. The Centralized Traffic Control system has a safety feature
that makes it impossible for a dispatcher to suddenly take away a permissive
route for a train after the engineer comes in sight of the signal. Although
the dispatcher can take away the displayed permissive signal and flash a
restrictive signal "in the face" of the crew, the switches controlling the
track route cannot change if the engineer lacks sufficient time to comply
with the changed signal. The system is so wired that the dispatcher cannot
set up conflicting moves. Single-track installations are so connected that
if a train fails to make a stop specified by a signal, other signals in
advance of the train are automatically displayed to stop a train that may be
approaching on the same track from the opposite direction.
Also, the CTC
interpretations of written train orders--misunderstandings that have
accounted for numerous collisions on single track.
Since Centralized Traffic Control is used principally
on single-track sections, some railroads have converted double-track
sections to single-track layouts.
Reduced maintenance-of-way costs and
accelerated freight train schedules resulted. Use of CTC frequently permits
an increase in train density, because the delay formerly caused by wait-and-
meet orders is reduced. With CTC, a dispatcher is frequently able to get
two trains by a given point without stopping either one, by keeping one
moving through a siding while the other passes on the main track.
siding is long enough or if a single track branches for several kilometers
into a double track, a dispatcher may likewise have a train pass another in
the same direction without reducing the speed of either. A nonstop meet can
be fully appreciated only by crews who in past years waited for hours for
other trains, or by dispatchers who were once required to dictate several
complicated train orders to do what CTC may often get done with less delay
by the dispatcher's moving two or three controls.