This postcard shows a downtown train platform of the Chicago Subway with an escalator leading to Adams and Jackson.
From the back of the postcard:
Every latest improvement in Automatic Signal and Train control Devices has been installed. An interlocking block system of signal reduces the human equation to the minimum. Signals operate at specific timed periods and permit no train to pass a "block" until the line is clear.----------Safety of patrons was the major concern of the designers and builders of the Chicago Subway. The Signal and Train Control System is the latest type. New safety features were specially developed for the Escalators.