According to today's WSJ, the summer temperatures in the Tube are getting out of hand. Heat levels in the stations and on the trains have been building ever since they opened, and now "regularly exceed 30 degrees C or 86 F, the legal limit for transporting cattle, pigs and sheep in the UK."
And ironically, when they put AC on the train cars, it ends up heating up the stations. Train drivers are threatening to possibly strike when temps get up around 96 F.