> Most of the tube tunnels have above ground sections, so a hybrid idea is to use air conditioning in the trains when above ground, and while above ground to cool a block of “phase change media”, or water to you and me, into an ice pack. When underground, the heat that would be dumped in the tunnels is absorbed by the ice-pack until it has returned to water.

> Whether this can be viable is still being looked at, bearing in mind that they already struggle to fit air conditioning units into tube trains, finding space for the ice blocks is going to be even more of a headache. And not to forget that the extra weight means more energy needed to drive the trains, driving up running costs.

This can be worked around, do the chilling on the wayside, not on the train! At each station, run chillers that can reject waste heat on the ground -- and chill a nontoxic liquid glycol/water mixture to -40 C. Commercial equipment exists to do this already. Have air/liquid heat exchangers on each EMU, along with glycol storage tanks, a pump, and sensors to keep track of the temperature/volume of the glycol in each tank. On the roof of the EMU, install large-diameter quick-mating liquid connectors, along with fiducial marks. At each station, wayside equipment uses computer vision to locate the fiducials on the EMU, mates with the connectors, does a pressure test to verify the integrity of the connection (squirting glycol is a no-no), pumps out all the warmed glycol (and replaces it with cold stuff), and disconnects. This can be done during the dwell time if the connectors and refill tubing are of large diameter.

Cooling loads are on the order of 50 kilowatts per EMU and inter-station times are on the order of 10 minutes, which means 30MJ per EMU. The ending temperature of the glycol will be on the order of 10C (you need a temperature differential to ensure heat flows from the glycol to the air), its initial temperature will be -40C -- a temperature difference of 50K. Glycol/water mixtures have a specific heat of around 3.2kJ/(kg * K), so we have:

30 MJ = (3.2 kJ / kg * K) * mass * 50 K

leading to a mass on the order of 200 kg, which is quite tolerable for a rail vehicle. The tanks for the glycol can be spread around the car and can be arbitrarily shaped (as long as fluid can be circulated and offloaded) to deal with other constraints. There's no phase changes involved, which makes the heat exchange work non-annoying; there's just liquid glycol and air. EMUs don't need to haul around an air-cooled chiller, all the equipment on the EMU is reliable, does not consume much electricity, and is extremely tolerant of vibration and the harsh environmental conditions aboard a rail vehicle.

If you want to reduce mass further and are willing to accept some more complexity, it might be sensical to use a small chiller on each EMU that uses the glycol for heat-rejection. What does this give you? It means that you can still generate a constant chilled water temperature of 10C, but let the end temperature of the glycol go above 10C -- and more temperature range on the heat storage fluid means more heat energy can be dumped into it. When the glycol temperature gets above 10C, turn on the heat pump to create chilled water at 10C, and reject heat into the glycol (stop before it boils). Liquid/refrigerant heat exchangers are much smaller than air/refrigerant exchangers, so if your compressor isn't obscenely heavy, you can likely save some weight. If you can use the glycol from 10C (when heat won't passively flow from the glycol to the air) to 60C (a reasonable condenser temperature) -- that's another 50K worth of temperature difference, which means our 200kg load of glycol can be cut in half.