Unions and Labour have raised warnings about jobs and safety on the railways and Tube in London amid fears that thousands of posts could be axed and ticket offices closed.
London Underground said it did not recognise a figure of 6,000 job losses, saying such a cutback would be "completely unfeasible" when it planned to run more trains in future and continue to staff all its stations.
But the Transport Salaried Staffs Association (TSSA) said up to 6,000 rail and Tube jobs could be axed over the next six years and all 278 Tube ticket offices closed under secret plans being drawn up by Transport for London (TfL).
The first 2,000 could could go in the next two years with Tube ticket offices replaced by travel centres, mainly at major stations such as Waterloo and Euston, said the union.
Managing director of LU and London Rail, Mike Brown said: "We are investing in London Underground to support jobs and growth in London and across the UK.
"We are committed to running more trains and that all Tube stations will continue to be staffed in future, with staff visible and available to help our customers.
"Learning lessons from the successful London 2012 Games, we are looking at how we can improve the service to our customers, while delivering the best possible value for fare and taxpayers' money.
"In recent years, we have successfully reduced costs and at the same time improved services. The Tube is now carrying more passengers, more safely and reliably than ever before. Any changes we propose to the way we staff our services in future will be discussed with our staff first."
TSSA general secretary Manuel Cortes said: "The mass ticket offices closures are just the start. We expect 6,000 jobs to be axed by 2020. We have a deal with TfL which clearly rules out compulsory redundancies. If Mayor Boris Johnson tears that up, all bets are off."
Shadow London Minister Sadiq Khan said: "These secret plans will have a devastating effect on the daily commute. Every Londoner has had to depend on staffed ticket offices when the machines are out of order or their Oyster card has stopped working. Under the Mayor's plans you will now have nowhere to turn in these everyday situations."