Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 31st was a station on the Chicago Transit Authority's South Side Main Line, which is now part of the Green Line. The station was located at 31st Street near State Street in the Douglas neighborhood of Chicago. 31st was situated south of 29th and north of 33rd. 31st opened on June 6, 1892 and closed on July 31, 1949. Chicago Transit Authority, also known as CTA, is the operator of mass transit within the City of Chicago, Illinois and some of its surrounding suburbs.