Not all psychopaths are killers and most usually operate alone, but John Duffy and David Mulcahy are a rare exception. Lifelong friends since their schooldays in Haverstock, north London, they were responsible for a four-year spree of rape and murder across London during the 1980s. They became known as the Railway Killers, due to their targeting of lone women around railway stations. It was this behavioural consistency that would eventually lead to their capture.
