Transport for London (TFL) operatives deal with poorly ventilated tube tunnels and temperatures as high as 47 degrees, as reported during the 2006 heatwave. However, with temperatures now surpassing this here’s which lines to avoid.
The hottest line appears to be the Bakerloo line according to TFL data (August 2017) but temperatures once hit 47 degrees during the 2006 heatwave.
London’s hottest lines
Waterloo and City 25.95C