Hyperloop is supposed to be open source, meaning we engineering types can talk about and solve issues. Here is one:
The cooling of the compressed air is to be supplied by water. The water is to be heated and turned to steam that is stored on the vehicle. So I ran some numbers....
The total water on board is 290 kilograms. It is to be turned into steam at 400 Kelivins. To get water to boil at that temperature it has to be pressurized to 260kPa. (Its not clear if it is intended to boil the water at elevated pressure, but as will be seen the issue I'm bringing up is worse the lower the pressure.) At 260 kPa and 400K stream has a density of 1.4 kg/cubic meter. 290 kg of this steam would require a storage tank with a volume of 207 cubic meters. At lower pressures the volume is greater.
I do not see room on the pod for such a tank. Given the pods frontal area of 1.3 square meters the tank would have to be 160 meters long, 4 to 5 times longer than the current design.
How to fix this? First, do not boil the water. Start with water-ice slush, say 70% ice, and let it cool by melting and heating to 400K. It will absorb only 25% or so of the heat as we would get by allowing it to boil, so we will have to carry 4 times as much water. But no steam tank is needed.
Next, see what we can avoid cooling. The bypass air may be left hot for example. Not cooling the bypass air has several advantages: The heat exchangers are smaller, we cut the water requirements in half, and when discharged it will create about twice as much thrust. The disadvantage is the bypass pipe will need to be larger and insulated from the passengers.