It gets hot in the London tube, and the trains aren't air conditioned. The trains barely fit in the tunnels so they can't bolt units on top, and since conventional air conditioners just move heat from one place to another, there is concern that putting them in trains would just make the tubes themselves hotter. Four years ago they even had a £100,000 competition for ideas to cool the trains; Julian Burgess', shown above, looks attractive.
Now they are going to try a quaint, 19th century idea: blowing air over blocks of ice mounted under the cars. 21st century twist: Refrigeration units will make the ice while the trains are outdoors, and let it melt while they are in the tunnels, cooling the occupants without warming the tunnels. The ice is acting as a kind of thermal storage battery. Downside: a lot of extra weight and extra energy to move it. ::Guardian and ::Daily Mail