Transport for London confirmed this week that air conditioned trains will be introduced on the Circle, District, Hammersmith and City and Metropolitan Lines by 2010.
But, other lines will have to wait for action due to the unique challenges posed by deeper tunnels which were designed to only provide space for trains.
As a result there is little space to introduce air conditioning units and nowhere to release the heat generated by air conditioning – which would pose a serious safety issue if a train became trapped for any length of time.
Mr Johnson said: “Cooling the tubes does remain a major challenge, especially on deeper lines such as the Northern and Piccadilly, where we will continue to strive for a solution to the problem.”
Other measures being introduced include installing 40 portable industrial fans at key stations, upgrading ventilation shafts on the Victoria Line and continuing the ground water cooling system at Victoria Tube Station.