The video footage shows a young child on a platform walking towards a train - and then they just disappear.
The video has been released by Transport for NSW to warn parents to watch their children while travelling on trains during the school holidays.
The incident happened at Sydenham station in February this year.
A train pulls in and the young child moves towards it, accompanied a parent carrying a baby in one arm and pushing a stroller with the other.
The child waits for others to leave the train and then tries to board but steps into the gap between the train and platform and plunges from view.
Bystanders frantically warn the driver not to move the train while others reach down and help pull the child back onto the platform.
As the train pulls away the child - who was uninjured - is comforted by the parent on the platform while station staff make sure they are okay.
Transport and Roads Minister Andrew Constance said accidents like this increase during school holidays.
"As a parent I know how hard it can be to juggle travel with young children, but a simple thing such as holding your child's hand when moving around stations and boarding a train can help prevent them from being seriously injured or worse," Mr Constance said.
Falling into the gap between the train and the platform was the most common incident, followed by accidents involving Opal barriers, and incidents of children slipping, tripping, or falling around train stations.
Sydney Trains Chief Executive Howard Collins said the incidents are more common at busy stations.
The most important message I'd like to share with parents is please don't rush around stations or platforms," Mr Collins said.
"We now have more trains services than ever before, so the next train is only a few minutes away," Mr Collins said.