Told by police that nothing further could be done without the alleged victim coming forward, Kathy Yu then uploaded one of the images she had taken, plus details of what she saw, to the UBC Confessions Facebook page, hoping to encourage the young woman involved to come forward.
'I saw you moving away from him'
"I saw the guy that was sexually harassing you," the post reads.
"He was feeling and rubbing the back of your right leg and butt with his left hand, moving up and down. We were on the Richmond-Brighouse to Waterfront SkyTrain around the Aberdeen and Bridgeport station.
"I saw you moving away from him," Yu writes. "I could tell that you were visibly uncomfortable."
The post continues to describe the young woman and her alleged assailant, and explains that Yu had the police stop the man.
"The police took all his information and I filed a complaint," Yu writes. "So if you can identify yourself, the policeman said he will press charges since he has all the information on file."
Metro Vancouver Transit Police confirmed the details of Yu's post to CBC News.
Suspect known to police
"The witness in this incident did exactly what we hope people will do," said transit police spokesperson Anne Drennan.
"If we know who [the alleged victim] is and can get a statement from her, we will help her through this process."
Drennan told CBC News the suspect is known to police and police later released a statement, asking for the woman to come forward.
Though applauding Yu for bringing the incident forward to police, Samantha Grey of Vancouver Rape Relief says it's important to understand there are reasons why women don't always report assaults.
"Women are up against a lot," she told CBC News. "And people have to keep in mind that it's not easy to come forward."
Photo, witness should be enough
Grey said the witness and the photo should be enough to lay charges.
"Stop putting the onus on the women to come forward about male violence," she said.
"The police have eyewitnesses, I imagine more than one. They have a photo of what the man did and I would imagine they have video from the SkyTrain that they can gather, and they can make a case against this man regardless of whether or not the woman comes forward."
CBC news tried to contact Yu for comment, but messages were not returned.