March 27, 1948 - service between Howard Street and Skokie replaced by buses. Only the Chicago North Shore & Milwaukee Railroad would use this trackage until its discontinuance in 1963, CTA service would resume the following year as the "Skokie Swift".
April 5, 1948 - CTA first introduces "A" and "B" skip stop service on Lake Street route. At the same time, Lake Street service discontinued to terminal at Market Street (now Wacker Drive).
July 31, 1949 - North-South services revised to structure similar to present structure, including adoption of "A" and "B" skip stop service. Service discontinued to Congress Street terminal and to North Water Street terminal.
February 25, 1951 - Dearborn Street Subway opens, connecting Logan Square and Humboldt Park branches with downtown, terminating at La Salle/Congress.
December 9, 1951 - service on Westchester extension replaced by buses, western terminal for Garfield Park line reverts to Des Plaines Avenue in Forest Park.
February 3, 1952 - service on Douglas branch west of 54th Avenue replaced by buses.
May 4, 1952 - service discontinued on Humboldt Park branch.
September 20, 1953 - street level operation replaces elevated line over a 2 1/2 mile portion of the Garfield Park line, enabling construction of the Congress (now Eisenhower) Expressway. This resulted in CTA no longer being able to accommodate the Chicago Aurora & Elgin Railroad trains, forcing passengers of the interurban line to transfer to and from CTA trains at Des Plaines Avenue, and eventually leading to the demise of the interurban.
January 29, 1954 - service discontinued on Normal Park branch.
April 4, 1954 - Douglas branch trains rerouted to Loop via Lake Street route.
October 11, 1955 - Garfield Park line trackage extended through Wells Street (formerly Fifth Avenue) terminal to connect with Loop elevated, replacing old Loop connection at Tower 8 (Wells/Van Buren).
July 3, 1957 - Chicago Aurora & Elgin Railroad ceases operations.
October 7, 1957 - service discontinued on Stockyards branch.
December 1, 1957 - service discontinued on Kenwood branch.
June 22, 1958 - Congress line opens in median strip of what now is the Eisenhower Expressway, replacing Garfield Park line. Route linked to Dearborn Street Subway at La Salle/Congress station, trains routed through to Logan Square. Douglas branch linked with Congress line.
October 28, 1962 - street level operation at west end of Lake Street line replaced by elevated operation, sharing right of way with Chicago and North Western (now Union Pacific) Railroad. Overhead trolley operation thus eliminated on the Lake Street line.
January 21, 1963 - Chicago North Shore & Milwaukee Railroad ceases operations.
April 20, 1964 - Skokie Swift service begins nonstop between Howard Street and Dempster Street, using trackage formerly owned by Chicago North Shore & Milwaukee Railroad until its demise.
May 6, 1969 - Englewood branch extended to new terminal at Ashland Avenue, 1/4 mile west of previous terminal at Loomis Blvd.
September 28, 1969 - service begins in median strip of Dan Ryan Expressway. Lake Street line routed through to Dan Ryan line via Loop elevated. Loop All Stations route introduced, circling Loop clockwise.
February 1, 1970 - service extended northwest of Logan Square to Jefferson Park, using median strip of Kennedy Expressway.
November 8, 1973 - overhead trolley wire on Evanston route replaced with third rail. Prior to then, trolley poles were raised and lowered at South Blvd. stop.
September 30, 1977 - Loop All Stations route discontinued.
March 4, 1982 - due to structural defects in bridge over Illinois Central Railroad, service on Jackson Park branch suspended south of 61st Street.
December 12, 1982 - service restored on Jackson Park branch as far as University Avenue stop. The defective bridge would later be demolished.
February 27, 1983 - first segment of O'Hare extension opens from Jefferson Park to River Road (now Rosemont), using the Kennedy Expressway median strip.
September 3, 1984 - final segment opens from River Road to O'Hare Airport.
February 21, 1993 - new connection opens between Dan Ryan line and State Street Subway, Dan Ryan trains routed through to Howard Street forming "Red Line". Englewood and Jackson Park elevated lines, previously linked with Howard Street, become linked with Lake Street line via Loop elevated, forming "Green Line". New roller signs on trains and route maps conform to new color coding system adopted by CTA for its rapid transit routes.
October 31, 1993 - new "Orange Line" opens between Loop and Midway Airport, utilizing existing railroad rights of way and new elevated structures.
January 9, 1994 - Lake-Englewood/Jackson Park "Green Line" closes for extensive renovation project lasting more than 2 years.
April 28, 1995 - last day of "A" and "B" skip stop service, by then running rush hours only on the Howard and O'Hare segments.
May 12, 1996 - Green Line reopens after renovation project lasting more than 2 years. Jackson Park trains terminate at Cottage Grove Avenue while future of elevated segment east of there remained undecided. That elevated segment was finally demolished in 1997.
September 13, 2004 - Yellow Line (Skokie Swift) segment converted from overhead catenary to third rail, standardizing electrification over entire system.
June 25, 2006 - Cermak branch of Blue Line (Douglas) becomes new Pink Line, disconnected from rest of Blue Line, and using routing previously used between 1954 and 1958 via Lake Street line and Loop elevated. Line would use former "Paulina Connector" between Harrison Street and Lake Street, which had not seen revenue service since 1958, and had only existed as the only physical connection between the Blue Line and the rest of the rapid transit system. Pink Line initially introduced "experimentally", with certain rush hour trains continuing to use Blue Line routing between 54th Avenue and O'Hare.
April 27, 2008 - Rush hour Blue Line trains via Cermak discontinued. Officially this was a change considered as part of the Pink Line "experiment", but the change would later be adopted as permanent.
April 30, 2012 - Oakton station opens as first intermediate station on Yellow Line (Skokie Swift).
May 19, 2013 - Red Line Dan Ryan segment closes for renovation project lasting 5 months. Red Line trains diverted instead via Green Line route to Ashland/63rd, with actual Green Line trains only operating to Cottage Grove/63rd.
October 20, 2013 - Red Line Dan Ryan segment reopens after 5 month renovation project.