Pauline Tantost, 24, filed a complaint with the STM last week over the incident, which she said began when she pulled out a $5 bill to pay for the $3 fare.
STM spokeswoman Amelie Regis said an internal inquiry was taking place and the driver and other agency employees would be asked for their version of events.
Tantost told The (Montreal) Gazette she was just trying to get home with her son, Xavier, after a lengthy visit to Montreal Children's Hospital last Friday. She boarded the bus at 10 p.m. but didn't have the required $3. She did have a $5 bill, but the driver refused to take it and warned her she could be ticketed by STM inspectors if she stayed aboard without a ticket.
Tantost refused to disembark and, sure enough, STM inspectors soon met the bus and asked to see her ticket. They then escorted mother and son to the sidewalk and left them there with a $219 citation.
"I left the bus on my own because I felt bad, I couldn't stop crying, so I didn't want everyone seeing me like that," Tantost told the newspaper.
Although STM said it wasn't the driver who called the inspectors, Wayne Larsen, another passenger who witnessed the scene, told The Gazette, "To me, it looked like it was an ambush because these guys got on and targeted her right away."