permit passenger trains to load at stations.
Although these tracks are
often thought to be reserved for passenger trains only, many dispatchers
permit their express trains and fast freights to use them if they can make
passenger-train running time. The current of traffic, defined in paragraph
1.8b, can be more rigidly enforced on a four-track system because of the two
tracks in each direction. If a dispatcher lets a fast freight out on the
high-speed track and it has trouble or fails to progress as expected, he can
have it switched bark to the low-speed one. As the following subparagraphs
point out, a distinction may or may not be made between the standards of the
high- and low-speed tracks.
a. Some railroads make a strict distinction between the standards of
the two types of track. On the low-speed track, lighter rail is used and
less ballast and subballast maintenance performed.
Tie renewal may be
considerably less frequent than on the high-speed track.
The tracks are
generally designated either freight or passenger, and interchangeability of
types of trains is not permitted except in emergencies. Continual operation
of heavy freights over passenger tracks requires constant maintenance work,
to keep them in the condition required to give maximum riding comfort.
b. Many railroads make no distinction between the standards of the two
types of track; the weight of the rail is identical, and the ties and
subgrade are the same. The terms "high speed" and "low speed" are used to
describe these tracks, but the timetable would refer to them as Nos. 1, 2,
3, and 4. Generally, the only time it would be necessary to run against the
current of traffic would be when both tracks in one direction were blocked
CENTRALIZED TRAFFIC CONTROL
Operating a rail division controlled with Centralized Traffic Control
(CTC) is much simpler than by the other methods. The dispatcher, who may be
many kilometers from the point where a train crew wants to enter the main
track, merely flips a switch on his CTC panel that opens a yard switch and
lights a proceed signal for the train.
A modern CTC panel is shown in
figure 1.5. The dispatcher does not advise the train crew of his plans nor
are written orders necessary. Tracks have signals facing in each direction,
and there is no established current of traffic in CTC-controlled areas.
This gives the dispatcher complete flexibility of train movements; he may
run trains on any track in either direction. Regardless of the class of a
train, it can continue to move against or ahead of trains of a superior
class as long as a signal tells it to do