A federal report says air quality testing found soot concentrations were higher on the platforms at Chicago's Union Station than on the streets outside the commuter station.
The dirtiest air in the station was found to be at 5 p.m., when rush-hour trains are packed inside. The EPA says it hopes to work with commuter rails to reduce particulate pollution.
Amtrak and Metra say they're continuing to work on additional improvements. The agencies have installed more efficient air filters on passenger cars and switched to cleaner fuel.
The U.S. Environmental Protection Agency released a report Thursday detailing the finding during the summer test of air quality. EPA regional administrator Susan Hedman declined to speculate on the agency's actions if voluntary efforts fail.