John Duffy, 40, originally sentenced in 1988, also pleaded guilty to six conspiracies to rape and two burglaries between 1975 and 1986 in London and Hertfordshire.
The Old Bailey was told he committed three rapes on Hampstead Heath in July and November 1984 and March 1985 near his home in West Hampstead, north London.
Duffy also admitted raping Ann Lock, a secretary, in Hertfordshire in May 1986. The judge ordered him to be found not guilty to her murder because of insufficient evidence.
The rape conspiracies cover October 1982 to November 1986 and centre on Hampstead and Highgate in north London.
Duffy's extensive knowledge of the railway and underground system helped him to stalk his victims and many of his crimes were committed near stations.
The fresh charges follow interviews with police in Whitemoor prison, Cambridgeshire, where Duffy is being held.
Sentencing on the fresh charges was adjourned until after Duffy appears for the prosecution in another trial.Reuse content