Three passengers have been praised for crawling underneath a Sydney train to rescue a trapped child this morning.

Key points: It had been the toddler's first time riding a train — and the last for a while, her mother said

It had been the toddler's first time riding a train — and the last for a while, her mother said The incident happened at Wolli Creek station shortly after 9:00am this morning

The incident happened at Wolli Creek station shortly after 9:00am this morning One witness said the mother was screaming that her daughter was under the train

The young girl fell under the train at Wolli Creek Station in Sydney's south about 9.15am.

She slipped through the gap between the carriage and platform as she tried to step off the train right after boarding.

The driver was immediately alerted and told not to move the train.

As her mother became distressed, three men jumped off the platform and began looking for the girl under the carriages.

As other commuters tried to comfort her, the mother was told the toddler had been found near the front of the train.

"It's OK, they've got her," one passenger was heard saying in a video posted online.

The young man who found the girl was then filmed handing her over to her mother, reassuring her "she's fine, she's fine".

A Sydney Trains employee then thanked the man and shook his hand.

The mother and toddler embrace after she is rescued by the man in the black shirt. ( Supplied )

The mother said it was the first time her daughter had been on a train and it "might be the last for a while".

"She freaked out and tried to fling herself back off the train," the mother was heard saying in the video.

Joseph Mourtada, 36, filmed the moment the mother and daughter were reunited, and said he was relieved there were no injuries.

Mr Mourtada was walking up the stairs to the platform when he saw the commotion and the distressed mother "shaking".

"She was screaming that her girl was under the train," he said.

"And I hear all of this noise and people asking the guards to stop. It was already stopped.

"Everything happened very quickly."

Chief customer officer for Sydney Trains Suzanne Holden thanked staff and customers for helping in such a distressing situation.

"Thankfully the toddler was unharmed, but incidents such as this serve as an important reminder that the gap between the train and the platform can vary across the network, and our staff are always happy to help if you require assistance," she said.

The gap between the platform and train is not the same distance at every station, and can be impacted by the curve of the rail line, the type of train and other variables.