"The warmest parts of the Underground network have been revealed in a
map compiled by Transport for London (TfL)."
Full story from BBC News online:
http://news.bbc.co.uk/1/hi/8218059.stm

Actual map can be seen either linked to from the above story, or
directly via:
http://tinyurl.com/Tube-heat-map

I don't think it's quite complete, looking at the Northern line -
"Data not available / selected" seems a little cryptic (well, "data
not available" is self-explanatory, but "not selected" is somewhat
less clear).