via Associated Press A man sits on a London underground train as he wears a protective mask, in London, Tuesday, May 12, 2020. (AP Photo/Alberto Pezzali)Temperatures on the London Underground reached almost 40C during the recent heatwave, making it the hottest June day on record. According to Bloomberg UK, the network of underground trains can be 5°C hotter than the high temperature on the street. This heat builds up – per Rail Magazine, the average tube temperature in 1900 was 14°C, compared to today’s 20-25°C. AdvertisementNot all tube stations have adequate ventilation, either, meaning hot air has nowhere to escape.