A girl of about five shocked passengers on the MTR on Tuesday night as she squatted and urinated on a train before alighting with her mother, who appeared to do little to stop her.
The incident occurred at about 8.15pm on an eastbound train on the Island Line, said one passenger, a Mr R. Dunn, who declined to give his first name.
The woman and the girl were travelling with a boy who appeared to be about six years old. The three, who were heard speaking Putonghua, got off the train at Causeway Bay station. The incident comes after a video clip showing a couple allowing their child to defaecate in an MTR station went viral in April.
Dunn said the girl just pulled down her trousers and crouched to urinate.
"I turned around and saw [the girl] squatting by the vertical hand rail. She was around five and p***ing right in front of me on the MTR. It lasted about a minute and it smelled horrible," he said.
Dunn said the woman, who appeared to be the girl's mother, did little to stop her. "The mother was just talking casually. She did not seem upset."
Dunn said his attention was initially caught when the boy started to eat a sausage roll. He tapped the woman on her shoulder and pointed at a sign showing eating was prohibited and the woman stopped the boy. But when he turned around, he saw the girl urinating.
He said passengers moved away, but remained calm.
"People were shocked. Like did that really just happen?" said Dunn. "You hear about it, but I never thought it would just happen in front of me. "But worst of all, they made an insignificant attempt to wipe it up and hurried off at Causeway Bay."
Two of the passengers had handed the woman tissues to help clean up the urine. The woman dabbed at the floor before alighting with the children.
A series of photos and a video clip taken by Dunn, who works in advertising, show a trail of liquid spreading across the grey floor of the carriage beneath a girl.
A woman carrying shopping bags and a pack of tissues is seen crouching to wipe it up.
"It's horrific and I believe people should know about it," said Dunn, who moved to the city three years ago. "I wasn't born in Hong Kong, but I also wasn't born yesterday. And as Hong Kong is my second home, I don't like seeing anyone soil it, let alone p*** on it."
An MTR spokesman said it had followed up with Island Line stations and that it had not received any passenger requests for assistance regarding the case.
"MTR strives to provide a clean and comfortable travelling environment for its passengers, and in order to achieve this, it needs their support and cooperation," he said.
Public toilets were available in 41 of the MTR's 84 stations and passengers could also contact station staff to use staff toilets if they needed assistance.
The spokesman said passengers should contact station staff to report any cases of nuisance behaviour so that staff could take immediate follow-up action.