ficiency were beyond question.

The application of such a system to the New York subway involved an
elaboration of detail not before attempted upon a railway line of
similar length, and the contract for its installation is believed to
be the largest single order ever given to a signal manufacturing
company.

In the application of an automatic block system to an electric railway
where the rails are used for the return circuit of the propulsion
current, it is necessary to modify the system as usually applied to a
steam railway and introduce a track circuit control that will not be
injuriously influenced by the propulsion current. This had been
successfully accomplished for moderately heavy electric railway
traffic in the Boston elevated installation, which was the first
electric railway to adopt a complete automatic block signal system
with track circuit control.

The New York subway operation, however, contemplated traffic of
unprecedented density and consequent magnitude of the electric
currents employed, and experience with existing track circuit control
systems led to the conclusion that some modification in apparatus was
essential to prevent occasional traffic delays.

The proposed operation contemplates a possible maximum of two tracks
loaded with local trains at one minute intervals, and two tracks with
eight car express trains at two minute intervals, the latter class of
trains requiring at times as much as 2,000 horse power for each train
in motion. It is readily seen, then, that combinations of trains in
motion may at certain times occur which will throw enormous demands
for power upon a given section of the road. The electricity conveying
this power flows back through the track rails to the power station and
in so doing is subject to a "drop" or loss in the rails which varies
in amount according to the power demands. This causes disturbances in
the signal-track circuit in proportion to the amount of "drop," and it
was believed that under the extreme condition above mentioned the
ordinary form of track circuit might prove unreliable and cause delay
to traffic. A solution of the difficulty was suggested, consisting in
the employment of a current in the signal track circuit which would
have such characteristic differences from that used to propel the
trains as would operate selectively upon an apparatus which would in
turn control the signal. Alternating current supplied this want on
account of its inductive