I was on the central line with my friend going to Ealing Broadway from the centre of London. At some point a man got on and sat directly in front of us and began staring at us in a way which made me uncomfortable. 3 stops from Ealing Broadway, the rest of the people in the tube carriage left the tube and the doors closed leaving just me, my friend and him. As soon as the doors had closed he began to unbutton his trousers and the proceeded to masturbate while staring at us both. We both froze and just looked away until the next stop when we could get off. For months afterwards I struggled to ride the tube and had a recurring nightmare about the incident. The police linked him to a similar attack a couple of weeks later and released the cctv images of him from that incident (since the cctv from our incident was overriding itself for the one hour that we needed it). Unfortunately they never found the man although I am hopeful that they will because a little over a year later, I was called in to view a line-up for a suspect they had pulled in. Sadly this turned out to be the wrong man.