Temperatures on London Underground lines have reached sweltering levels, with some exceeding 35°C (95°F) during a recent heatwave, according to tests conducted by the Daily Mail. The Victoria Line was found to be the hottest, reaching 35.2°C, a temperature significantly higher than the 30°C limit set by government legislation for transporting cattle.
As ambient temperatures in London climbed to 36°C, Senior Science & Technology Reporter Wiliam Hunter used a thermometer to measure conditions on various Tube lines and bus routes. The Victoria Line, which serves popular stations including Brixton, Oxford Circus, and Victoria, registered 35.2°C, approximately 4°C hotter than the outside temperature. Humidity levels within carriages ranged from 60% to 80%.
Transport for London (TfL) acknowledges the Victoria Line as its 'official' hottest line, with previous station measurements recorded at 31.1°C in August 2024. However, the recent on-train measurements were considerably higher.
Other lines also experienced intense heat. The Piccadilly Line followed closely at 34.4°C. This line is awaiting new air-conditioned trains, a part of a £3.4 billion investment, which are not expected before December at the earliest. The Central Line recorded 34°C, and the Bakerloo Line reached 33.3°C. The Northern Line, while comparatively better, was still a warm 33.1°C and noted for its high humidity, between 75% and 80% while in motion.
Currently, only four London Underground lines are equipped with air conditioning: the Circle, District, Hammersmith & City, and Metropolitan Lines. These lines utilize the larger, more modern S-Stock trains and recorded temperatures around 28.2°C, offering a more comfortable commute.
Bus routes were also tested, with one bus route (the 328) reaching 34°C, nearly 3°C warmer than the outside temperature at the time of measurement.
The Elizabeth Line, though not technically part of the London Underground, emerged as the coolest option, with temperatures recorded at 25.6°C. This line features powerful air conditioning systems and stations designed for efficient heat removal. This temperature was nearly 10°C cooler than the hottest Tube line and about 6°C cooler than the outside air.
Historical context suggests that the London Underground's underground location once provided a refuge from heat. However, rising temperatures of the surrounding clay soil, which absorbs heat from trains, and poor ventilation in older, deeper lines have contributed to increasing temperatures over time. TfL engineers have been working on cooling solutions for over two decades without a definitive solution.
Claire Mann, TfL's Chief Operating Officer, advised commuters to consider the necessity of their journeys during extreme heat warnings. Health experts, such as Professor Jesus Lizana from the University of Oxford, have noted the UK's vulnerability to heatwaves due to infrastructure designed for cooler climates, warning that even moderate temperatures can have significant impacts.