June 21, 1958 - last streetcar route on Wentworth replaced with buses.
September 28, 1969 - opening of Dan Ryan rapid transit line results in many changes, mostly on far south side of Chicago.
February 1, 1970 - extension of rapid transit line from Logan Square to Jefferson Park results in many changes on northwest side of Chicago.
March 24, 1973 - last trolleybus routes on Pulaski and Cicero replaced with motor buses.
August 5, 1973/September 9, 1973 - two rounds of service cuts due to first financial crisis at CTA. CTA, and public transit in general, could no longer be self sufficient. The Regional Transportation Authority (RTA) would be formed the following year, as a means of subsidizing area transit.
September 13, 1981/January 3, 1982 - two rounds of service cuts due to second financial crisis at CTA. A political deal had resulted in a loss of state subsidy to RTA. Subsidy was later restored, after RTA was required to replace its board and undergo corporate restructuring.
February 27, 1983 - extension of rapid transit line from Jefferson Park to River Road (now Rosemont) results in many changes on northwest side of Chicago. Final segment from River Road to O'Hare Airport would open following year.
November 7, 1993 - opening of rapid transit "Orange Line" to Midway Airport one week earlier, results in many changes on southwest side of Chicago. Several southwest side express routes experimentally continued operation six months longer, meeting demands by skeptics and a few vocal riders resistant to changing their commuting habits. Most express routes were indeed discontinued six months later.
October 5, 1997/December 28, 1997/April 26, 1998 - three rounds of service cuts as CTA sought further efficiencies with declining transit ridership over the years. After 1997, CTA successfully reversed the ridership declines by improving existing services with potential, and adding a few new services where there was market demand.
August 31, 2003 - North and South Lake Shore routes restructured and segmented, with several new routes added, enabling direct express service between downtown and more locations.
June 18, 2006 - several route extensions and limited stop services introduced on Chicago's west side. The following week, a portion of the rapid transit Blue Line would become the Pink Line.
February 7, 2010 - a poor economy makes it necessary to discontinue nearly all limited stop bus routes introduced in recent years, and reduce service hours and frequencies on most routes.
December 16, 2012 - CTA implemented a "crowding reduction plan", redistributing its capacity and discontinuing several lightly used routes.