Cook County Suburbs
By using the Google satellite view, one can see the present status of that roundhouse or turntable.
Roundhouses were usually located near the downtown Chicago passenger terminals, and at railroad freight yards. These are where locomotives were turned and serviced. In addition, turntables were formerly located in various suburbs, where commuter trains turned. With the adoption of push pull trains on what is now Metra, these turntables are no longer necessary.
Not all locations had turntables. Either locomotives were generally bidirectional switchers not requiring turning, or wye tracks existed for turning trains or locomotives.
Additional information is from the Historic Aerials Web site, and from the Train Watcher's Guide To Chicago, by John Szwajkart.