I never got why the heat in the tube was not being used as a heat source. You could extract the heat and supply it to surrounding buildings at a cost, thereby cooling the tube. The tech is readily available. It would be a win win situation. Plus it'd be be very environmentally friendly.
The problem is that the heat isn't a point source, it's diffused across hundreds of kilometers of tunnel. The amount of infrastructure you'd have to build to extract it at scale would simply cost way too much. It's cheaper for any reasonable timeframe to simply continue burning natural gas at the surface for heat than to invest all of this into infrastructure at a very long-term ROI.
Thermodynamics also makes this really hard. The tube is at most 40 degrees celsius. The air outside is perhaps 20 degrees celsius in summer. There's just not enough of a temperature gradient to run any kind of efficient engine.
It could be useful for low-grade thermal applications -- heating (or pre-heating) hot-water supplies, or space heat.
Even simply ducting warm air to street level for outdoor dining (in winter), presuming (against other information, I'm aware) that the ducting could be provided. If you're wasting the heat anyway, put it to some use prior to final venting.
