It was just another train journey, a paper in tow, salty snacks, a caffeine-laden drink – the usual kit. But as I settled in and started to scroll aimlessly through my phone, suddenly, out-of-the-blue, a photograph of someone’s genitalia flashed up on my screen unannounced. My eyes must’ve burst blood vessels as I gawped and looked around to find the perpetrator in what felt like an X-rated whodunnit.