More than 30 people were injured, though none seriously, when the Chicago Transit Authority train crashed early on the morning of March 24.
The train operator, who investigators and the CTA did not identify, had been running trains for 60 days and admitted dozing off before the crash, according to the National Transportation Safety Board. She had also admitted to overrunning a station in February.
The transit authority may terminate an operator for two serious safety violations under its contract with the union for those workers and she was terminated for two violations, CTA spokeswoman Tammy Chase said.
Robert Kelly, president of the Amalgamated Transit Union Local 308 that represents the operators, could not be reached immediately for comment.
The train was traveling at about 26 mph (42 kph) when it entered the station, a normal speed, and tripped an emergency braking system beside the track that failed to stop it before the impact, according to the NTSB.
The CTA said it would lower the speed limit for trains entering the station to 15 mph (24 kph) and move up the trip switches to engage emergency braking earlier on trains exceeding the limit.
The crash in March was the second in recent months involving an apparently out-of-control CTA train. In September, an unmanned CTA train ran onto active tracks and collided with a standing train at a suburban Chicago station during the morning rush hour, injuring at least 33 people.
After a deadly derailment of a New York commuter train late last year, an engineer told investigators he became dazed and lost focus before the train, traveling at nearly three times the speed limit, hurtled off the tracks near the end of its run. Four people were killed and more than 70 were injured.