"Goalie of the Century" wrote

Surely using energy to pump heat out of the trains into the tunnels will
just increase the temperature there. Has a solution to the problem of
cooling the system as a whole been found? I have heard groundwater
mentioned as a possible heat sink but building and running a full system
would be horribly expensive.

Sub-surface stock (Met/District/Circle/H&C) will be air-conditioned. Tube
stock will only have air-conditioning in the driver's cabs. Trials of
groundwater colling are being carried out at Victoria.

Peter