Speaking on the sixth anniversary of Network Rail taking charge of Britain's rail infrastructure, Sir Ian McAllister, chairman, said: "Network Rail has delivered substantial improvements in the safety, reliability and efficiency of the railway over the past six years.
"Safety is at record levels, train punctuality is now above 90%, and we are on track to reduce costs by around 30% by the next AGM. I have led the Board for over six years now and a great deal has been achieved in that time.
"As the company enters a new phase in its development, it is entirely appropriate that a new chairman helps take the company forward. The next challenge is for Network Rail to help deliver an expanded and enhanced railway to meet the growing demand from passengers and freight customers. My role now will be to help find a replacement and see through a smooth transition."
McAllister was instrumental in setting up the Network Rail bid team in November 2001 and played a key role in taking Railtrack out of administration in October 2002.
In addition, Ron Henderson, group finance director, has announced his intention to retire.
Network Rail chief executive Iain Coucher, chief executive, said: "Ian and Ron have been outstanding members of the leadership team. Ian has been part of Network Rail from its inception, and has led the company from the substantial problems it inherited, to become a successful company, with both train punctuality and safety at record levels. His contribution has been enormous."
Three new executive directors were appointed to the board two weeks ago: Operations and customer service director Robin Gisby, infrastructure investment director Simon Kirby and planning and regulation director Paul Plummer