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…
That Is Insane!!
Rail needs a complete rethink of design from the ground up. There are certainly vast efficiencies from rolling steel wheels on steel rails. But I've seen little evidence of light-weighting and indeed active resistance to steps like making cars using composites, which would save enormous amounts of energy and cost over the lifetime, as well as reduce maintenance costs. Yet their approach to everything seems to be sticking with an approach of 'brute force and ignorance'. Sure that works in the 19th century when you're just starting to put some carbon in iron and make it stronger than pig iron, but materials are sooo far past that now, yet it's still the same old approach. (&yes, I strongly support the Chesterton's Fence checking but this seems way beyond that)