Complex sentence: In 1993, CTA began to eliminate skip-stop service when it switched the southern branches of the Red and Green Lines; after this point, Green Line trains stopped at all stations, and Red Line trains stopped at all stations south of Harrison.
Split into simpler sentences: In 1993, the CTA began to eliminate skip-stop service when it switched the southern branches of the Red and Green Lines. After this point, Green Line trains stopped at all stations, and Red Line trains stopped at all stations south of Harrison.