Engineering trials have begun for the first fleet of air conditioned Tube trains, London Underground has said.
The 191 new 'S stock' trains, being tested on the network's sub-surface lines, are longer and will increase capacity by nearly 20%.
The trains will be introduced on the Metropolitan Line in September 2010 and then rolled out onto the Circle, Hammersmith & City and District lines.
The Tube is the oldest underground railway network in the world.
London Underground is also trying to develop ways of installing air conditioning within the network's deep lines, such as the Northern and Victoria.
London's Mayor Boris Johnson described the trials of the new air conditioning trains as "a miracle of modern technology", but admitted it would raise expectations.
"The difficulty is of course that it will ramp up the public pressure on us to deliver air-conditioning on the deep lines as well."
The Tube began service in 1863. It runs along 249 miles (402km) of track.