In the 1940's and 1950's, the two subway lines were constructed with automatic block signals and mechanical trip train stops. These automatic signals also included a time delay feature at curves, inclines, and other areas with speed restrictions. The time delay feature would normally have a signal at yellow. And as a train approached one of these signals, the signal would not clear until several seconds later. If the train was operating at a safe enough speed, enough time would elapse and the signal would clear before the train passed it. If the train was operating too fast, the signal would still be yellow and trip the automatic train stop function.
The automatic block signal system was also installed on the Congress route, which opened in 1958, and on the elevated segment northwest to Logan Square. In 1967, the CTA's first cab signal system was installed on the Lake Street line. Cab signals were also installed on the new Dan Ryan and Jefferson Park extensions, opened in 1969 and 1970. Shortly after the opening of the Jefferson Park extension, cab signals were extended over much of the Logan square elevated segment, due to problems with the cab signals cutting in and out at the incline between the elevated and the new subway segments.
During the 1970's, cab signals were installed on all remaining which did not already have any type of automatic signals. Completed on the Douglas line in 1974, the North-South and Evanston and Skokie lines in 1975, and the Ravenswood line in 1976. The Skokie Swift line previously had the original North Shore Line automatic block signals, prior to replacement with cab signals.
Beginning with the 1983 extension from Jefferson Park to O'Hare Airport, new signal systems included bidirectional capability.
In 2001, the automatic block signals and mechanical trip train stops on the State Street subway line were replaced with cab signals, leaving only the Dearborn subway and Congress route with the automatic block signals and mechanical trip train stops. Between 2007 and 2009, those remaining signal systems were replaced with cab signals.
Just as with automatic block signals, a line with cab signals is divided into blocks. Wayside equipment at the end of each block sends a coded electrical current through the rails, which is detected by coils above the rails at the front of a train. The train receives the code from wayside equipment ahead of it. The train's axles shunt that coded current, so following trains do not receive it. The absence of a code is interpreted as a red signal, which is appropriate anyway for a following train in this fail safe system.
The CTA uses a high frequency cab signal system which functions without insulated rail joints separating the blocks. The current used to detect trains is at either of three frequencies (1300 hz, 1950 hz, 3060 hz). The blocks alternate among these three frequencies, which is how the need for insulated joints is eliminated.
A fourth frequency (4550 hz) is used to send a code the train for the cab signal. The cab signal current pulses at different rates according to the signal conditions. A "270 rate" code pulses at 270 times a minute, and is used for a green-55 mph cab signal indictation, the maximum speed. There is also a "180 rate" for a yellow-35 mph signal, a "120 rate" for a yellow-25 mph signal, and a "75 rate" for a yellow-15 mph signal. No code received is interpreted as a red stop-and-proceed at 15 mph signal. And a "410 rate" is the "cab signal cutout" code, telling the cab signal equipment that the train is about to exit cab signal territory.
The design of the cab signal/speedometer display has changed over the years, but the displays have always conveyed the same basic information. Besides the usual green yellow and red lights, the CTA's displays utilize light indicators near the speedometer. These light indicators, in conjunction with the actual speedometer, provide a convenient indication of the actual vs. allowable speed. When the train exceeds the allowable speed, a high pitched warning beep gives the driver 2 1/2 seconds to apply the brakes, and avoid an automatic brake application.
For spacing of trains, the yellow-35 mph signal is the normal yellow signal used in advance of an occupied block, where the signal is red. All three yellow cab signal indications can be used for speed restrictions at curves and inclines. In addition, temporary speed restrictions can be imposed by setting selector switches inside the wayside equipment cabinets.