Andrew Schad was on his way home Sunday, traveling north on the Red Line after a late lunch downtown, when the train began to fill with smoke.

Some seated beside him began to cough and choke, and Schad put up the rim of his collar to cover his nose and mouth. By the time the train reached the next stop at the Clark/Division station, so much smoke had consumed the underground station that passengers were fleeing for the exits.

"We were given no instruction. We didn't really know what was going on," said Schad, 23. "I never saw flames, but there was a lot of smoke. Visibility was really bad."

Investigators worked late into the night Sunday to figure out what had sparked an extra-alarm fire on an underground track that sent 19 people to hospitals for smoke inhalation and respiratory problems.

Five people were transported with serious injuries, Chicago Fire Department spokesman Richard Rosado said. The injured included a 10-year-old boy who was being held overnight at Children's Memorial Hospital for smoke inhalation. The extent of their injuries was not known Sunday night.

"The smoke was so thick you couldn't see across the aisle," said passenger Dillon Johnson, 23. "We all started to sit down on the floor where the smoke wasn't as bad."

Fire officials said railroad ties caught fire just before 5 p.m. on the northbound track between the Red Line stops at Chicago Avenue and Clark/Division. Black smoke could be seen billowing from several subway grates and vents in the area, including near Gibsons Bar and Steakhouse on Rush Street. Red Line trains and several bus routes were redirected while firefighters fought the small underground blaze.

Fire Department spokesman Larry Langford said it's unclear what sparked the fire, but that railroad ties occasionally catch fire during the summer heat.

"It's more common on the elevated trains," Langford said. "It's rare on the subway. But it does happen occasionally."

By the time firefighters had controlled the fire at 6:18 p.m., hundreds of people were milling near the entrance to the Clark/Division stop and paramedics were taking some away in ambulances and assisting others with oxygen masks.

By 8 p.m., service had been restored to both the Red Line's northbound and southbound routes. It's not likely the damage will affect Monday morning commuters, CTA spokeswoman Catherine Hosinski said.

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