The fire broke out just before 5 p.m. on the tracks between stops at Chicago Avenue and Clark and Division.
Although service was interrupted, it was restored fully later Sunday night. Monday morning commuters are warned, however, that there is slow zone, 15 mph, between the Clark/Division and Chicago stop, as the investigation continues.
Interestingly, there was a fire two years ago in the same area in the southbound Red Line tunnel. This time, the problem was in the northbound tunnel.
As the fire burned, smoke poured from street level-vents as emergency crews treated people suffering from smoke inhalation. In all, 19 people were taken to hospitals. Five of those people were in serious to critical condition.
Hundreds of passengers evacuated through Red Lines stations at Division and Chicago. The worst smoke was closest to the Division Street stop.
" The smoke was really harsh, and everyone's face was getting all black with ashes," passenger Sang Shin told ABC7 Chicago.
Home video from inside one of the trains showed what passengers faced trying to breathe and find their ways out through the thick smoke.
"We started smelling something burning. It smelled like rubber burning, and we could see smoke coming into the car," said Aliess Kime, also a passenger.
Some passengers witnessed the fire, too.
"One of the cars caught on fire, and it was full of smoke," said Richard McReynolds. "Smoke was coming in the train."
"When we did move, there were flames almost to the top of the car," said Brian Drier.
"As they drove the train to let us off, they drove right by the flames. The flames went way up from off the track," Lora Wallera said.
CTA and fire officials say it was actually rail ties three blocks south of Division at Bellevue that burned.
It was not clear how the fire ignited, but lubricants on the ties may have contributed to the problem.
"It was the ties that were involved. With all the grease that's on there, it causes a horrific amount of smoke, which a lot of people saw," said Deputy Fire Comm. Gene Ryan of the Chicago Fire Department.
"We don't want to say for sure what the cause is yet until it has been fully investigated, but there is grease used to lubricate the rails, and it's possible that was involved," said CTA's Noelle Gaffney.
One hour and 20 minutes later, the smoke cleared, but some passengers say they're done with train travel.
"It's really scary, and I'm telling you, I won't go in this train again," said passenger Peter Kime.
As investigators try to determine the cause of the fire, a CTA spokeswoman says the grease on the ties is made to not catch fire. However, debris can mix in with it, and that can be flammable.
None of the injuries was life-threatening.