I think you were incredibly brave doing what you did. However, for the sake of the child, I think you should also call the police and report exactly what you saw. Try to remember any details that might help identify the woman -- age, dress, where she got on and off the bus, time, did she call the child by a first name, brand of stroller, etc.
Also, make sure to call CHILD PROTECTIVE SERVICES in your city. They take such reports very seriously. Remember, the behavior abusive parents exhibit in public is generally only the tip of the iceberg, and what such children have to go through at home is often ten times worse. In addition, if this woman is ever cited for even more serious behavior in the future, a previous report on file at the police and at child protective services (from your report) will make sure she can't talk her way out then (e.g;., claim it was a household accident, etc).
An 18 month old toddler in a town close to us DIED recently from being hit repeatedly on the head, and the reason was that child protective services allowed him to be sent back home to his mother from the hospital where he had been reported by the doctors because of suspected abuse. The mother claimed a household accident and they let him go home because there were no prior reports on her. A few weeks later he died from blunt force trauma to his head. Autopsy showed he had been beaten black and blue repeatedly since his early BABY days, and had even been ANALLY RAPED by the mother's boyfriend. This child was 18 months old, and what he had to go through before finally dying was unspeakable.
Please report this.