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CTA bulletin 8-511—76, issued December 8, 1976, and reissued on February 7, 1977, (see appendix C) requires that a train be immediately stopped on receipt of a red cab signal and that it not proceed on either a steady red or a flashing red cab signal without the operator first obtaining permission of the central controller. However, the bulletin did not supersede or annul any part of CTA training brochure "Cab Control Signalling." There are several locations on the CTA system where the only cab signal to proceed is a flashing red indication because the present track facilities cannot be adapted to give other signal aspects. Those locations are governed by special instructions permanently posted at those points. There are no such locations on the West-South Line.

The central controller has operational control over all trains. He monitors the passage of trains past specific points by means of time-synchronized pen charts, and directs train movements in the event of trouble. He contacts the train by means of the trainphone system.

The cab signals and ATC receive command signals from audio frequency transmitters along the track at the exits of signal blocks. In the "loop" area the signal blocks average about 300 feet. The signal system in the "loop" is designed so that there is at least one unoccupied signal block behind an occupied block to provide necessary stopping distance for an approaching train. The approach to the unoccupied red signal block is governed by a "yellow 15" cab signal aspect.