[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.  Interlocking Working
                                       Machines.  Levers. 
MAIN LINE.

City Hall,                                 3               32
Spring Street,                             2               10
14th Street,                               2               16
18th Street,                               1                4
42d Street,                                2               15
72d Street                                 2               15
96th Street                                2               19

WEST SIDE BRANCH.

100th Street,                              1                6
103d Street,                               1                6
110th Street,                              2               12
116th Street,                              2               12
Manhattan Viaduct,                         1               12
137th Street,                              2               17
145th Street,                              2               19
Dyckman Street,                            1               12
216th Street,                              1               14

EAST SIDE BRANCH.

135th Street,                              2                6
Lenox Junction,                            1                7
145th Street,                              1                9
Lenox Avenue Yard,                         1               35
Third and Westchester Avenue Junction,     1               13
St. Anna Avenue,                           1               24
Freeman Street,                            1               12
176th Street,                              2               66
——­             ——­
Total,                               37              393

The total number of signals, both block and interlocking, is as follows: