A fleet of faster, longer trains is to be brought into service on four Tube lines as part of a £3.1bn upgrade.
The new trains will start running in 2009
Transport boss Peter Hendy said new signals and track would also be installed on the Metropolitan, Circle, District and Hammersmith & City Lines.
The air-conditioned trains will start operating on the Metropolitan Line in 2009 and on all the lines by 2013.
Maintenance firm Metronet, which is to carry out the work, pledged increased capacity and shorter journey times.
The firm said the upgrade would deliver 21% more trains per hour on the busiest sections of the upgraded lines.
It also said there would be 25% more room for passengers and journey times would be cut by 15%.
Metronet's Mark Loader said: "This new and enlarged fleet of 190 air-conditioned trains will be bigger, faster, more reliable and run more frequently, carrying more people than ever before."
The carriages will be air-conditioned
The Circle and Hammersmith and City Lines are expected to have the new trains operating by 2012 with the District Line following the year after.
London Mayor Ken Livingstone said: "This upgrade will be felt by passengers every day, who will benefit from air-conditioning and extra space on the trains.
"It will also deliver major energy efficiencies which will reduce energy costs and make a positive contribution towards reducing London's CO2 use and tackling climate change."