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.
John Duffy and David Mulcahy (both born 1959) are two British rapists and serial killers who together attacked numerous women and children at railway stations in the south of England during the 1980s. They are known as the Railway Rapists and the Railway Killers.
In 1982, a woman (KJ) was raped by two men near Hampstead Heath railway station; and eighteen more were attacked over the next year. Even more occurred during 1984, and then three were raped on the same night in 1985 in Hendon. West London Police set up an urgent investigation to try to find the perpetrators, called "Operation Hart".

On 29 December 1985, Alison Day, 19 was on her way to meet her boyfriend at work in Hackney Wick. She was followed off a train at Hackney Wick station by Duffy and Mulcahy who grabbed her and she was repeatedly raped. She was then strangled with a ligature and tourniquet. Her body was sunk into the River Lea using discarded cobbles (granite sets). Metropolitan Police in East London set up a further separate investigation - Operation Lea.

Police further stepped up their search for the attacker who had been nicknamed by the press the "Railway Rapist". The death of Alison Day changed this name to the Railway Killer, a tag reinforced by the rape and murder of 15-year-old Maartje Tamboezer in West Horsley in Surrey on 17 April 1986. As well as rape and strangulation, Maartje's body was set on fire. Surrey Police set up Operation Bluebell. Meanwhile the Alison Day murder enquiry was taken over by Detective Superintendent Charles Farquhar (a highly experienced East London Murder investigator) and he linked that murder with the previous Railway rapes. He then drew a link with the murder of Maartje Tamboezer when he spotted that a belt and twig in a scene photo were the parts of a tourniquet ligature. A month later on 18 May 1986, Anne Lock, 29, a secretary at London Weekend Television was abducted and murdered after she alighted from a train at Brookmans Park railway station, Hertfordshire. This heralded the first multi Police Force murder enquiry (Operation Trinity) since the badly executed Yorkshire Ripper enquiry. It was the first such investigation to utilise basic computers and an early version of HOLMES (Home Office Large Major Enquiry System).

The name of Duffy, a martial arts exponent and former railway carpenter, was identified by Det. Supt. John Hurst as a suspect among thousands of other names. He was a known sex offender, having been previously convicted of the rape of his wife.