On my way to the hospital to pick up my medication, crossing from line 5 to line 12 subway. I noticed a woman in front of me going up the escalator, she looked good and i guessed she must have been about 30 or so. Didn't really take much more notice and thought about my next direction, looked for the entrance of line 12 and headed in that direction. Funny enough, this woman was in front of me and going in the same direction, my attention was drawn to the way that she was walking because it wasn't quite right. Anyway, we head to the next escalator, she steps on and i step on about three steps behind her. All of a sudden when i stepped onto the escalator a man steps on at the same time and stand on the step directly in front of me. I thought to myself "what an idiot" because as we moved it meant that we were actually touching, even when i moved to the back of the step, there still wasn't enough room. It wasn't until we all got off and walking down the corridor that i noticed he was following the woman, he was following her every step behind her. When she walked around a person, he would do the same, I also noticed that he had his phone in his hand and holding it at the same height as her arse, he was obviously videoing her backside as she walked. I called my wife and explained to her what was happen and the man stopped to look at the sign to see which way he wanted to go, the woman headed to the gates to get out of the subway. I was explaining to my wife very loud in order to shame this man (which was wrong), and the man suddenly changed direction and followed the woman out. My wife notified the guards and my wife ran after the woman, she noticed that he was doing it again. My wife videoed him doing this because something wasn't right. She caught up with the woman and asked if she knew the man behind her, when they looked back, the man suddenly changed direction back to the subway. they all got stopped by the police and the man was questioned, it was then that i found out that the woman was in fact a young girl of 16 years. she was very stressed and frightened but she had done nothing wrong and we tried to comfort her, my wife and the girl swapped wechats and the police took details of everyone.