The heat from the trains etc. heats the surrounding clay over a period of years. You can add less heat, or you can remove heat from the surrounding clay, or bo

The heat from the trains etc. heats the surrounding clay over a period of years. You can add less heat, or you can remove heat from the surrounding clay, or both. Removing: Run ventilation on high during winter and keep temps quite low in the tunnels. Run ventilation on high on cooler nights. Install cooling tubes in the surrounding clay and cool it directly. Either from above, or from the tunnel itself, a ground-source heat pump (geothermal heat pump) to pull heat from the clay. These can be powered by the cheapest available power, likely solar on sunny days in the future. Adding less: Upgrade motors to highest efficiency available. This could halve the waste heat from the motors. Regenerative braking: if it is too complex to put the power back on the grid, build large "electric kettles" and dump it into a vat of water with resistance heaters. The water vat could be part of a water main so it would be constantly refreshed, and result in slightly warmer water for water users. Instead of ice in the cars, cool brakes and motors that exceed 100C with water, by boiling the water. This absorbs terrific amounts of heat per kg water.