The fire occurred near the Logan Square stop at North Kedzie and North Milwaukee. Paramedics took at least two CTA passengers to the hospital for minor smoke inhalation.
Viewer-submitted video: Matthew Chomiczewski, of Chicago, e-mailed video of a foggy Logan Square subway station.
"It was just all fog. It smelled like burning wood. I do not know what else was down there," said rider Melissa Canfield.
"The firemen came down, about a dozen of them, and told us get out of the train. Get out on the street," said rider Oli Goldenstein.
"It was pretty scary. The train started filling up with smoke," said Neal Harper, rider.
"It was kind of difficult to see. And the smoke was kind of burning our eyes," said rider Eugene Heider.
Firefighters quickly put out the fire. It appears debris on the rails somehow caught fire.