The company said it would be looking at five routes north and west of London - Chiltern, East Coast, West Coast, Great Western and Midland Main Line.
It is likely that most, if not all, of any new lines will be high speed in what would amount to the largest track building programme since the 19th century.
Network Rail chief executive Iain Coucher said: "By 2025 many lines will be full-up, especially those running to and from the north and west of London. This will happen even after we have implemented the investment to boost current capacity.
"With popularity for rail growing, we have to start planning for the medium and long-term future today. We have to see how we can meet the capacity challenge and see what solutions - including potentially, that of new lines - are deliverable and affordable.
"This review, working in partnership with other players in the railway industry, will kick-start this process."
The rail operator said it had launched a tendering process to appoint a consultancy to assist its new-line study.
It is anticipated that Network Rail will announce the name of the successful consultancy firm by the end of July and that an initial report will be completed in summer 2009.