The creation of "entirely new rail lines" may be necessary to cope with train overcrowding on some routes over the next 20 years, Network Rail (NR) said today.
The situation was most pressing on some outer suburban routes from the south-west into London and between London and Ipswich, NR said.
A new approach would be needed to meet predicted future demand on "a small number of routes", with normal measures such as train lengthening, timetable changes and infrastructure enhancements possibly being too complex and costly.
"More extensive options - for example, the building of entirely new lines - may be needed," said NR, which published its 20-year strategy for London and south-east England services.
NR said the number of passengers travelling into London in the busiest hours was set to grow by more than a third by 2031.
Schemes such as the west-east, cross-London Crossrail project and the north-south, Thameslink project are already under way.
NR said that as long as these, and other projects were implemented in full, "overall peak capacity in the London suburban area will largely be able to cope with predicted passenger numbers in 2031, with the addition of some other key measures".
It said these other measures included extra commuter services between the Thames Valley and Paddington station in London, additional tracks on the Lea Valley line in east London and more trains on the Windsor lines into London's Waterloo station.
NR also recommended developing proposals for extending Crossrail, improving rail links to Heathrow and having another look at a previously-explored project - a Chelsea to Hackney line under London.
NR's planning and development director Paul Plummer said: "Some of the conclusions in this report are stark. This strategy should act as the starting point for a wider discussion, looking beyond rail planning to housing policy, the ability of the railway to drive regeneration and changes to how we live, work and travel."