d are distinctly in
advance of anything heretofore done in this direction.

[Illustration: REAR VIEW OF BLOCK SIGNAL POST, SHOWING TRANSFORMER AND
INSTRUMENT CASES WITH DOORS OPEN]

Block section lengths are governed by speed and interval between
trains. Overlap lengths are determined by the distance in which a
train can be stopped at a maximum speed. Usually the block section
length is the distance between signals, plus the overlap; but where
maximum traffic capacity is desired the block section length can be
reduced to the length of two overlaps, and this was the system adopted
for the Interborough. The three systems of blocking trains, with and
without overlaps, is shown diagramatically on page 143, where two
successive trains are shown at the minimum distances apart for
"clear" running for an assumed stopping distance of 800 feet. The
system adopted for the subway is shown in line "C," giving the least
headway of the three methods.

[Illustration: PNEUMATIC TRACK STOP, SHOWING STOP TRIGGER IN UPRIGHT
POSITION]

The length of the overlap was given very careful consideration by the
Interborough Rapid Transit Company, who instituted a series of tests
of braking power of trains; from these and others made by the
Pennsylvania Railroad Company, curves were computed so as to determine
the distance in which trains could be stopped at various rates of
speed on a level track, with corrections for rising and falling to
grades up to 2 per cent. Speed curves were then plotted for the trains
on the entire line, showing at each point the maximum possible speed,
with the gear ratio of the motors adopted. A joint consideration of
the speeds, braking efforts, and profile of the road were then used to
determine at each and every point on the line the minimum allowable
distance between trains, so that the train in the rear could be
stopped by the automatic application of the brakes before reaching a
train which might be standing at a signal in advance; in other words,
the length of the overlap section was determined by the local
conditions at each point.

In order to provide for adverse conditions the actual braking
distances was increased by 50 per cent.; for example, the braking
distance of a train moving 35 miles an hour is 465 feet, this would be
increased 50 per cent. and the overlap made not less than 697 feet.
With this length of overlap the home signals could be located 697 feet
apart, and the block section length would