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Something that is very cool (hah!) is that originally they used the coolness of the deep tube lines as a selling point in summer[1], but over the last century the surrounding clay has absorbed so much heat that the ambient temperature is something like 30° [1] https://www.ianvisits.co.uk/articles/cooling-the-tube-engine...

That is very cool. So if the capacity of the under ground to extract heat from the underground, is something that can diminish on a decades level timescale, what does that say about the longevity of ground source heat pumps I wonder?

It depends on the composition of the ground and how heat pumps are used. In places like Canada with wild temperature variations over the course of the year heatpumps have the potential to be used as both a means to cool buildings in the summer, and then heat them in the winter with that stored heat.

They should honestly close it down for those days. It sounds like a torture method straight out of Guantanamo.

You can avoid deep tube lines for many journeys. Elizabeth line, Thameslink, overground, circle, district are all air conditioned. I've definitely taken slightly longer routes to avoid the deep tube lines when it's been hot before.