lied only
while the machines are running.

Two novel safety devices having to do especially with the signaling
may be here described. The first is an emergency train stop. It is
designed to place in the hands of station attendants, or others, the
emergency control of signals. The protection afforded is similar in
principle to the emergency brake handle found in all passenger cars,
but operates to warn all trains of an extraneous danger condition. It
has been shown in electric railroading that an accident to apparatus,
perhaps of slight moment, may cause an unreasoning panic, on account
of which passengers may wander on adjoining tracks in face of
approaching trains. To provide as perfectly as practicable for such
conditions, it has been arranged to loop the control of signals into
an emergency box set in a conspicuous position in each station
platform. The pushing of a button on this box, similar to that of the
fire-alarm signal, will set all signals immediately adjacent to
stations in the face of trains approaching, so that all traffic may be
stopped until the danger condition is removed.

The second safety appliance is the "section break" protection. This
consists of a special emergency signal placed in advance of each
separate section of the third rail; that is, at points where trains
move from a section fed by one sub-station to that fed by another.
Under such conditions the contact shoes of the train temporarily span
the break in the third rail. In case of a serious overload or ground
on one section, the train-wiring would momentarily act as a feeder for
the section, and thus possibly blow the train fuses and cause delay.
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 switch