There was an item on the news last night about overheating on the tube - but
in the stations, not on the trains. Engineers at Victoria (and later on at
other stations which get very hot) will use the water that is pumped out of
the ground (which would otherwise flood the station) to cool the ambient air
in a heat exchanger. They quoted reductions of temperatures of about 5 deg
C - to 25 deg C.

But none of this addresses the problem of the trains themselves getting
unbearably hot, which mainly happens because so few windows on modern stock
can open so you don't get a draught through the train.