There are a number of reasons the London Underground doesn't have air-conditioning on most lines – one being that the Tube system is simply very old. The oldest tunnels were built in the Victorian era and are just about big enough for the trains themselves, according to Engineering & Technology.
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Why doesn't the London tube have air conditioning? The trains on the District, Hammersmith & City and the Metropolitan lines do have air conditioning. These lines are sub surface lines with larger tunnels and trains than the deeper tube lines, thus they have the space for the equipment and to expel the hot air.
Wear light clothing. Even if you think you might need to wear a jacket, when you are on the hot Tube, it's likely you'll want to take it off immediately. Heavy, dark, thick clothing will make you more likely to sweat, and hold on to the sweat your body does produce – keeping you hot and clammy for longer.
The heat in the tunnels is largely generated by the trains, with a small amount coming from station equipment and passengers. Around 79% is absorbed by the tunnels' walls, 10% is removed by ventilation, and the other 11% remains in the tunnels.
In most tubes the cathode is indirectly heated by a filament wire. The power transformer sends 6.3 volts of alternating current (AC) through twisted green heater wires to power the Champ's pilot light and the V1 and V2 filament heaters.