<P> Later, after assuming control of the "L", the CTA introduced an express service known as the A / B skip - stop service . Under this service, trains were designated as either "A" or "B" trains, and stations were alternately designated as "A" stations or "B" stations, with heavily used stations designated as both--"AB". "A" trains would only stop at "A" or "AB" stations, and "B" trains would only stop at "B" or "AB" stations . Station signage carried the station's skip - stop letter and was also color - coded by skip - stop type; "A" stations had red signage, "B" stations had green signage, and "AB" stations had blue signage . The system was designed to speed up lines by having trains skip stations while still allowing for frequent service at the heavily used "AB" stations . CTA first implemented A / B skip - stop service on the Lake Street Line (now part of the Green Line) in 1948, and the service proved effective as travel times were cut by a third . By the 1950s, the service was used throughout the system . All lines used the A / B skip - stop service between the 1950s and the 1990s with the exception of the Evanston and Skokie lines, which were suburban - only lines and did not justify skip - stop service . Also, the Congress and Douglas branches of what later became the Blue Line were designated as "A" and "B" respectively, as were the Englewood ("A") and Jackson Park ("B") branches of what later became the Green Line, so individual stops were not skipped while trains were serving those branches . As time went by, the time periods which employed skip - stop service gradually decreased, as the waits at "A" and "B" stations became increasingly longer during non-peak service . By the 1990s, use of the A / B skip - stop system was only used during rush hour service . Another problem was that trains skipping stations to save time still could not pass the train that was directly ahead, so skipping stations was not advantageous in all regards . In 1993, the CTA began to eliminate skip - stop service when it switched the southern branches of the Red and Green Lines to all stops . After this point, Green Line trains on the entire line stopped at all stations, and Red Line trains stopped at all stations south of Harrison . The elimination of A / B skip - stop service continued with the opening of the all - stop Orange Line and the conversion of the Brown Line to all - stop service . On April 28, 1995, the A / B skip - stop system was completely eliminated with the transfer of the O'Hare branch of the Blue Line and the Howard branch of the Red Line to all - stop service . The removal of skip - stop service resulted in some increases in travel times, and greatly increased ridership at former "A" and "B" stations . </P> <P> The first air - conditioned cars were introduced in 1964, the last pre-World War II cars were retired in 1973 . New lines were built in expressway medians, a technique implemented in Chicago and followed by other cities worldwide . The Congress branch, built in the median of the Eisenhower Expressway, replaced the Garfield Park "L" in 1958 . The Dan Ryan line, built in the median of the Dan Ryan Expressway, opened on September 28, 1969, followed by an extension of the Milwaukee elevated into the Kennedy Expressway in 1970 . </P> <P> At 102.8 miles (165.4 km), the Chicago "L" is the fourth largest heavy rail rapid transit system in the United States in terms of route mileage . It runs over a total of 224.1 miles (360.7 km) of track . Ridership had been remarkably stable for nearly 40 years after the CTA takeover despite declining mass transit usage nationwide, with an average of 594,000 riders boarding each weekday in 1960 and 577,000 in 1985 . Due to the Loop Flood in 1992, ridership was at 418,000 that year because CTA was forced to suspend operation for several weeks in both the State and Dearborn subways, used by the most heavily traveled lines . </P> <P> Ridership is healthy and growth continues, but it has not been uniformly distributed . Use of North Side lines is heavy and continues to grow, while that of West Side and South Side lines tend to remain stable . Ridership on the North Side Brown Line, for instance, has increased 83% since 1979, necessitating a station reconstruction project to accommodate even longer trains . </P>

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