The small amount of available space in the subway made it necessary to design a special form of the signal itself.  Clearances would not permit of a “position” signal indication, and, further, a position signal purely was not suitable for the lighting conditions of the subway.  A color signal was therefore adopted conforming to the adopted rules of the American Railway Association.  It consists of an iron case fitted with two white lenses, the upper being the home signal and the lower the distant.  Suitable colored glasses are mounted in slides which are operated by pneumatic cylinders placed in the base of the case.  Home and dwarf signals show a red light for the danger or “stop” indication.  Distant signals show a yellow light for the “caution” indication.  All signals show a green light for the “proceed” or clear position.  Signals in the subway are constantly lighted by two electric lights placed back of each white lens, so that the lighting will be at all times reliable.

On the elevated structure, semaphore signals of the usual type are used.  The signal lighting is supplied by a special alternating current circuit independent of the power and general lighting circuits.

A train stop or automatic stop of the Kinsman system is used at all block signals, and at many interlocking signals.  This is a device for automatically applying the air brakes to the train if it should pass a signal in the stop position.  This is an additional safeguard only to be brought into action when the danger indication has for any reason been disregarded, and insures the maintenance of the minimum distance between trains as provided by the overlaps established.

Great care has been given to the design, construction, and installation of the signal apparatus, so as to insure reliability of operation under the most adverse conditions, and to provide for accessibility to all the parts for convenience in maintenance.  The system for furnishing power to operate and control the signals consists of the following: 

Two 500-volt alternating current feed mains run the entire length of the signal system.  These mains are fed by seven direct-current motor-driven generators operated in multiple located in the various sub-power stations.  Any four of these machines are sufficient to supply the necessary current for operating the system.  Across these alternating mains are connected the primary coils of track transformers located at each signal, the secondaries of which supply current of about 10 volts to the rails of the track sections.  Across the rails at the opposite end of the section is connected the track relay, the moving element of which operates a contact.  This contact controls a local direct-current circuit operating, by compressed air, the signal and automatic train stop.

Direct current is furnished by two mains extending the length of the system, which are fed by eight sets of 16-volt storage batteries in duplicate.  These batteries are located in the subway at the various interlocking towers, and are charged by motor generators, one of which is placed at each set of batteries.  These motor generators are driven by direct current from the third rail and deliver direct current of 25 volts.