A fellow York Region Transit passenger recorded the video last Friday while at the Richmond Hill Viva station, near Yonge Street and Highway 7.
The nearly three-minute video shows a woman moaning in pain while a special constable holds her down with what appears to be his knees on her back.
York Region Transit says the video shows only a portion of a 30-minute interaction between the special constable and the woman. But police are now investigating the matter.
York Regional Police Const. Andy Pattenden said the transit system has called in police to investigate.
"York Regional Police is currently investigating the totality of the events that led up to the video that is seen by the public," he told CBC News on Monday.
"Right now, it's a special constable with York Region Transit that is under investigation with this incident. And right now it's in the hands of our investigators, so there really isn't much that we can say on the investigation or the details of it."
The special constable has been placed on administrative duty until the investigation is complete.
Click on the video player above to see a full report from the CBC's Ivy Cuervo.