Under the plans, blocks of ice would be hidden from view in refrigerated tanks stored underneath seats. Above ground, where temperatures are generally lower, the ice will simply be kept in its frozen state. But, when the train goes underground, the refrigeration will be switched off, allowing the ice to slowly melt. As this happens, air will be cooled by pumping it over the ice, which absorbs much of the heat.Presumably what they mean is that in tunnels hot air will be blown over cubes, melting the ice and cooling the air. Above ground, the ice will be re-frozen electrically, extracting the heat.
There's no space for such apparatus on the current trains, so as the article notes, it could be linked with the new 2012 Stock due on the line by 2014, which hasn't been designed yet. That is, assuming there's any truth in it.