The Victoria line never dropped below an average of 24C last year — even in winter — and exceeded 31C in August last year. The highest recorded monthly temperature was on the Central line, with an average of 31.3C last August.
Last August, average temperatures on the Central and Victoria lines both exceeded 31C — with the Central line a fraction hotter. The Bakerloo averaged 30.4C. Average annual temperatures across all Tube lines increased from 22.5C in 2021 to 23.8C last year, according to Transport for London.
The Victoria line, which never comes above ground for the duration of its 13 miles between Walthamstow Central and Brixton, attracted more than half of all complaints that TfL catalogued as “temperature — too hot”.
Of the 133 received by TfL since 2020, 69 have been about the Victoria, 19 about the Central and 11 about the Jubilee.
Between last September and November alone, there were 48 complaints about the heat on the Victoria line, despite passenger numbers being well below pre-pandemic levels.
The Waterloo & City and the Jubilee were the coolest lines, while the Metropolitan and Hammersmith & City attracted fewest complaints. TfL said air-conditioned trains were in use on 40 per cent of the Underground network. New trains due to be introduced on the Piccadilly line from 2025 will also have air conditioning.
But TfL said further efforts to keep the “deep-level” lines cool would not be possible without investment from the Government beyond next April.
It said the Victoria and Jubilee lines had “enhanced tunnel ventilation systems”, while industrial-sized fans and chiller units were being used to pump in cold air at other hotspots.
Mayor Sadiq Khan said London had looked to learn lessons from other world cities on how to cool the Tube.
He said: “Now 40 per cent of our Underground is cool and air-conditioned. We can make progress, and we are doing more innovation in relation to those Tube lines for which there is no air-conditioning.”
TfL said it had installed solar reflective material on train roofs and film on windows to minimise heat from the sun’s rays entering the carriages.
A TfL spokesperson said: “Air-conditioned Tube trains cover 40 per cent of the Underground network and there are enhanced tunnel ventilation systems on both the Victoria and Jubilee lines.
“We have 18 ventilation shafts fitted with heat extraction fans on the Victoria line and have installed a chiller system between Blackhorse Road and Walthamstow Central stations which pumps chilled air into a mid-tunnel shaft between these stations.
“Trains on the Victoria line also use regenerative braking which reduces the amount of heat generated by trains braking in tunnels.
“On older parts of the Tube network with fewer ventilation shafts, we have introduced a range of station cooling systems including industrial-sized fans and chiller units to pump in cold air.”