A mother watched in horror from a platform at Southern Cross Station as a peak-hour train left with her baby inside but, frighteningly, it's not as uncommon an event as you might like to think.

The mother and child spent four terrifying minutes separated on Friday afternoon, as the baby travelled to Flagstaff in the care of concerned and panicked passengers.

Addressing the incident after it emerged in the media on Tuesday, Metro Trains' safety boss Anthony Fewster revealed parent-child separations – with either the child on the train, or the parent – happened about once a fortnight.

"It's definitely too many," Mr Fewster said. "We want to keep people as safe as we possibly can [and] we've seen the trend go down in relation to train doors, etc, in the last six months.