I wonder if an extremely tall subterranean windcatcher [0] with its bottom at the top of a tunnel could passively cool the tunnel.

[0] https://en.wikipedia.org/wiki/Windcatcher

The catch here is the cost of the buildings you'd need to destroy in order to build the surface portion of the windcatcher. The deep underground lines run under heavily populated areas of London.

To build enough windcatchers to move the needle on tunnel temps, you'd need to buy many plots in one of the most expensive cities in the world.

These basically already exist around the network. But that simply isn’t even close to moving enough energy to make a difference.