The size, shape and proximity to multiple railroads makes this look to me like the leftover foundation of a long demolished roundhouse. Google maps show it clearly but Bing maps aerial and Birds eye view show it overgrown. MSR Maps topographic maps of area don't show it.
Info Requested, Demolished
By: bucsfan2010
Info on it here and pictures before it was demolished.
http://stlouispatina.blogspot.com/2009/02/old-roundhouse-east-st-louis.html