The incident unfolded at around 12.45pm on Tuesday at Narre Warren station, when the woman, believed to be in her 20s, sprung into action.
Sprinting onto the tracks, she was able to pluck the boy from the path of an oncoming train before returning safely to the platform.
While witnesses have described her actions as heroic and brave, questions regarding the whereabouts of the child's parents have remained unanswered.