In order, therefore, to prevent trains passing into a dangerously
overloaded section, an overload relay has been installed at each
section break to set a "stop" signal in the face of an approaching
train, which holds the train until the abnormal condition is removed.
[Illustration: THREE METHODS OF BLOCK SIGNALING]
[Illustration: DIAGRAM OF OVERLAPPING BLOCK SIGNAL SYSTEM
ILLUSTRATING POSSIBLE POSITIONS OF TRAINS RUNNING UNDER SAME]
[Sidenote: _Interlocking
System_]
The to-and-fro movement of a dense traffic on a four-track railway
requires a large amount of switching, especially when each movement is
complicated by junctions of two or more lines. Practically every
problem of trunk line train movement, including two, three, and
four-track operation, had to be provided for in the switching plants
of the subway. Further, the problem was complicated by the restricted
clearances and vision attendant upon tunnel construction. It was
estimated that the utmost flexibility of operation should be provided
for, and also that every movement be certain, quick, and safe.
All of the above, which are referred to in the briefest terms only,
demanded that all switching movements should be made through the
medium of power-operated interlocking plants. These plants in the
subway portions of the line are in all cases electro-pneumatic, while
in the elevated portions of the line mechanical interlocking has been,
in some cases, provided.
A list of the separate plants installed will be interesting, and is
given below:
Location.
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