The main cause of heat is the inefficiency of the trains. All the brake energy is converted to heat. The motors and controllers are pretty inefficient too - modern electric motors+controllers are 95-98% efficient, whereas most tube trains are more like 40-60% efficient. For example, to go slowly, they simply put a big resistor in the circuit and waste 50+% of the energy!!
Perhaps not coincidentally, the inefficiency also has a massive carbon and budget impact... Or it would, if the train system had to pay for the electricity it used - which it doesn't! Trains in the UK are given free unmetered electricity, so there is no reason to conserve energy. Hot tunnels are just an extreme symptom of that.
If they had to pay for usage, at todays wholesale electricity rates, it would cost nearly a billion pounds a year. Which is ~20% of their revenue! I can see why they don't want to pay that bill!
Perhaps not coincidentally, the inefficiency also has a massive carbon and budget impact... Or it would, if the train system had to pay for the electricity it used - which it doesn't! Trains in the UK are given free unmetered electricity, so there is no reason to conserve energy. Hot tunnels are just an extreme symptom of that.
If they had to pay for usage, at todays wholesale electricity rates, it would cost nearly a billion pounds a year. Which is ~20% of their revenue! I can see why they don't want to pay that bill!