The child, who was thrown onto the tracks at the Rogers Park Morse Red Line station Nov. 2, was not seriously hurt.
"It appears that what she's been reporting and testified to could have occurred at this time, again nothing faulty with the train," said Richard Rodriguez, CTA president. "They were operating as designed. We will now turn attention to the rail operator."
The train operator has been suspended without pay pending the investigation.
Police are working it as a non-criminal investigation. They have some physical evidence, but they don't yet have eyeball witnesses who actually saw what happened, and on an early evening Red Line run, police presume there had to have been witnesses.
Transit union officials say the doors show no damage.
On the wooden platform at the Morse stop, there were no discernible marks that one might theorize would have been left by a stroller being dragged outside the train.
The stroller itself, in a picture released by the union, doesn't appear to be badly damaged, but it is a lightweight, collapsible stroller, and it's not clear how or how much of it was stuck in the door.
"She appears to be very credible," said Lt. Dennis Walsh, Chicago police.
Police say they believe the mother's account of what happened, and they have some physical evidence to support it.
"There's paint from the rail on the south end of the train tracks that's on the stroller," said Lt. Walsh.
That paint would have come from a barricade at the end of the Morse train platform, indicating that the stroller hit it. The 22-month-old child was pitched to the ballast below before her mom and a man who raced to help lifted the toddler to safety.
But if the stroller was hanging outside the train, how did it avoid smashing into a similar barricade at the next station? And most perplexing, the stroller was eventually pulled inside the train and recovered without great damage several stops later. Police believe someone at some point pulled it inside the train.
Police said last week they were still investigating who pulled the stroller into the train car.
There is a police pod camera directly across from the Morse station, but the train would've blocked its view of the incident.
While the train operator remains suspended, which is standard CTA procedure, union leaders have long objected to that practice as tantamount to "guilty till proven innocent."