A train derailed and started a fire during the evening rush hour Tuesday, filling a subway tunnel with smoke and forcing dozens of soot-covered commuters to evacuate while critically injuring at least two, officials said. Terrorism was not suspected.

Evacuations began after the last car of an eight-car commuter train slipped off the tracks and material beneath the train caught fire, said Chicago Transit Authority President Frank Kruesi. Passengers were moved to safety within an hour.

At least 120 passengers were taken to hospitals, primarily for smoke inhalation, said Fire Department spokesman Larry Langford. Most were in good or fair condition, officials said, but at least one hospital reported two patients in critical condition.

The train had just left the downtown Clark and Lake station shortly after 5 p.m. when the operator realized there was a problem and stopped the train, Kruesi said. The operator called for power to the train to be cut, then led passengers out of the cars and through the smoky subway tunnel to an emergency exit, where they climbed to the street above.

Bomb and arson detectives were on the scene, but there was no evidence of foul play, Police Superintendent Philip Cline said.

Investigators were searching for the cause of the derailment and fire.

Service was shut down on a rail line that connects the city's West Side to O'Hare International Airport through downtown, said Chicago Transit Authority spokeswoman Ibis Antongiorgi.

Joel Johnson, 24, said he was on the train three cars from the back when "it felt like it jumped the line, and a fire started in the car behind me."

"People started running to the front. The whole place was filled with smoke," he said. "I could barely breathe."

Brenda Lopez, who was on the same car, said the train "dipped a little."

"I'm still kind of scared right now," she said. "You never expect it to happen on the train you're on."