Neillw001 wrote:

I've been on underground lines in other countries that get as hot or
hotter than London does in the summer and have never noticed any undue
heat. I think a lot of it has to do with the original constuctions
methods. Some places such as Budapest and Paris are cut and cover, with
the stations not far below the surface. But both these were constructed
not long after London and I've never found them to be as hot as parts
of the District/Circle in high summer. The Paris stations are quite
large, but many of the Budapest one are rather small.

Merseyrail is always pleasantly cool in summer, and I believe this is
in part down to groundwater - so the story on the BBC website about
cooling that may be helpful.

Neil