[Illustration:  VIEW UNDER CAR, SHOWING TRIGGER ON TRUCK IN POSITION TO ENGAGE WITH TRACK STOP]

The protection provided by this unique arrangement of signals is illustrated on page 143.  Three positions of train are shown: 

“A.”  MINIMUM distance between trains:  The first train has just passed the home signal, the second train is stopped by the home signal in the rear; if this train had failed to stop at this point, the automatic stop would have applied the air brake and the train would have had the overlap distance in which to stop before it could reach the rear of the train in advance; therefore, under the worst conditions, no train can get closer to the train in advance than the length of the overlap, and this is always a safe stopping distance.

“B.”  CAUTION distance between train:  The first train in same position as in “A,” the second train at the third home signal in the rear; this signal can be passed under caution, and this distance between trains is the caution distance, and is always equal to the length of the block section, or two overlaps.

“C.”  CLEAR distance between trains:  First train in same position as in “A,” second train at the fourth home signal in the rear; at this point both the home and distant signals are clear, and the distance between the trains is now the clear running distance; that is, when the trains are one block section plus an overlap apart they can move under clear signal, and this distance is used in determining the running schedule.  It will be noted in “C” that the first train has the following protection:  Home signals 1 and 2 in stop position, together with the automatic stop at signal 2 in position to stop a train, distant signal 1, 2, and 3 all at caution, or, in other words, a train that has stopped is always protected by two home signals in its rear, and by three caution signals, in addition to this an automatic stop placed at a safe stopping distance in the rear of the train.

[Illustration:  ELECTRO-PNEUMATIC INTERLOCKING MACHINE ON STATION PLATFORM]

[Illustration:  SPECIAL INTERLOCKING SIGNAL CABIN SOUTH OF BROOKLYN BRIDGE STATION]

[Sidenote:  Description of Block Signaling System]

The block signaling system as installed consists of automatic overlapping system above described applied to the two express tracks between City Hall and 96th Street, a distance of six and one-half miles, or thirteen miles of track; and to the third track between 96th and 145th Streets on the West Side branch, a distance of two and one-half miles.  This third track is placed between the two local tracks, and will be used for express traffic in both directions, trains moving toward the City Hall in the morning and in the opposite direction at night; also the two tracks from 145th Street to Dyckman Street, a distance of two and one-half miles, or five miles of track.  The total length of track protected by signals is twenty-four and one-half miles.