Firefighters say it was smoke caused by the train's brakes about 1:15 a.m.
"Couldn't see nothing. That's why they had to direct everybody to come up here," one commuter said.
"There was a lot of smoke, real dark smoke," said another rider.
"We made it off. It was no big deal. The fire department is excellent at their job. They have it under control, but for the people trying to go someplace, obviously, they're all stuck," said another rider.
Trains were rerouted to the elevated tracks for a time, but they were running normally Friday morning. No one was hurt.