It's around 2am on the Piccadilly Line and the Tube carriage is empty – apart from me and a man twice my age. His eyes never leave me during the 40-minute journey.Potbellied and with a menacing grin, he moves suddenly from the seat opposite, plonking himself down next to me. He's desperate to get my attention, talking at me over the deafening noise of the train.Unnerved, I get off before my stop. But he follows me and on to another near-empty train – this time the Jubilee Line towards Stratford. Again and again he tries to engage me in chat, staring at me and licking his lips.I finally manage to lose him somewhere between the platform and the escalators at North Greenwich station.Half an hour later, I'm on the platform at Green Park when another man starts trying to attract my attention, calling out to me. He's well dressed in a Barbour-style jacket with fashionable glasses. He beckons me to sit down next to him while we wait for the train.'Beauty needs a seat, now. Come sit down,' he says. I tentatively take up a place with two seats between us. He tells me he has a daughter who is my age.He keeps asking me where I live, and after failing to get an answer starts to threaten me. 'I will find the pub or restaurant next door to you. I'm going to come to look for you and I will find you.'He doesn't give up, repeatedly demanding my phone number. I tell him politely 'no' 20 times – I counted. Mimi Yates travelled across London's transport network at all hours of the day and night, secretly filming what happened to her and documenting how it felt to move around the capital as a young woman on her ownI board the train I've been waiting for. Laughing, he follows me into the same carriage, sitting down opposite me as he persists asking for my number. It's around 3am now and the carriage is busy but that doesn't deter him.'You have to give me your number, you have to. I am asking for yours. You have to meet up with me.'Then he reaches over and strokes my thigh. 'Please don't touch me,' I hear myself say.A girl and her partner see what's happening but say and do nothing.By the time I get home, it's almost 5am. I am badly shaken and film my reaction on my phone. 'My heart is still beating quite fast. I just don't think I expected it to be that bad.'This wasn't a normal night out. I had been working undercover for the Daily Mail's investigative series, Underground UK.Over two months earlier this year, I travelled across London's transport network at all hours of the day and night, secretly filming what happened to me and documenting how it felt to move around the capital as a young woman on her own.What I experienced has changed the way I view the city I love. I was verbally sexually harassed around eight times and physically assaulted once on four journeys in the course of my undercover investigation, at night and during the day