And because it's permanently warmed up, the long term consequence is the line becomes a health hazard and has to be closed for increasingly long periods.

When wet bulb > body temp people start getting heat stroke, which leads to fainting and potentially death - a bad look for a public transport system.

The likely remedy is to install gigantic refrigeration units in the ventilation shafts and pump in cold air. This will be hugely expensive to build and run.

But the alternative is a tube line that can't be used. So there may not be much choice.