The Chicago Tribune measured pollution levels inside trains, on platforms and at Union Station and Ogilvie Transportation Center. The newspaper's testing found the air inside train cars sometimes contained levels of diesel soot 72 times higher than on the streets outside.
The U.S. Environmental Protection Agency categorizes diesel exhaust as one of the most dangerous types of air pollution.
Metra officials told the tribune it is trying ways to cut pollution, given the agency's budget constraints.
The Tribune reports Metra is now planning a meeting with local, state and federal officials.