Police said the incident happened at the Bloor-Yonge subway station downtown at around 9 p.m.
A 39-year-old woman suffered serious but non-life-threatening injuries due to the fall but was not hit by the train, police said.
Police told Global News the woman managed to crawl underneath the covered part of the platform to avoid being struck by the train that came through the subway station.
Investigators said it is believed that this was a random incident and not targeted.
They said they do not believe that the suspect or the victim knew each other.
It is unclear what happened before the woman was pushed but police said they do not believe there was any altercation beforehand.
Several emergency crews responded to the incident and police are still looking for a female suspect.
The suspect is described as a woman with blond hair and an average build. She was last seen wearing a grey Levi shirt, a black jacket, black pants, white shoes, and a pink and grey toque and was carrying a bag.
Toronto police announced a 45-year-old woman had been arrested Monday evening.