Transit police have said the suspect is known to them, but charges can't be recommended without a complaint by the victim.
"I think I did everything I could, and I just really hope that she'll come forward," said Yu in an interview today.
"She should think about … there's many, many, many occasions where this is happening."
'He just started groping her'
Yu said the assault happened during the Thursday-morning rush hour between Aberdeen and Bridgeport stations in Richmond.
"I see his palm starting to face her and eventually it just ended up on her thigh, and he just started groping her," said Yu.
"I just was really angry."
Yu said she wanted to see if the woman was aware what was happening, so she made eye contact, mouthing the words, "Are you OK?"
The woman appeared embarrassed but didn't react further, said Yu.
"I saw her reaction to him and it really broke my heart. [I thought] this man can't get away with it, and he can't do it to anyone else."
She took a photo to report it to police and saw the man respond.
"He kind of moved his hand away. So you know then, I became very scared, because it seemed like this man knows what he's doing."
He got off at Bridgeport station and Yu followed him, yelling at transit police and pointing at the man.
Victim should decide on her own, say advocates
Yu would like to see the man she reported to police charged with the assault.
"He knows who I am," said Yu.
"My friends and family are very scared for my safety because the girl hasn't come forward yet, so the man is not charged."
Police put out a description Friday of the victim, trying to identify her in their investigation.
Women's advocacy groups said that was a mistake.
"I think, in cases of sexual assault, that it's very important to allow the woman to decide whether or not she wants to come forward," said Samantha Grey of Vancouver Rape Relief.
"The way that the criminal justice system is, it largely fails women … when they experience sexual assault."