[yev]

For those who also didn't know about liquid air: "Liquid air is air that has been cooled to very low temperatures (cryogenic temperatures), so that it has condensed into a pale blue mobile liquid. To protect it from room temperature, it must be kept in a vacuum insulated flask." (c) Wiki

Plan sounds great and very expensive.

[memracom]

The flask is for small amounts in a lab. An ore processing plant liquifies gases on an industrial scale and if the temperature is low enough, you can keep it in large vats like any other liquid. Of course I am suggesting something in between where you only make enough to keep the pipes full. Of course it might make sense to make it in big vats overnight and drain them down throughout the day.

This could be tested with a single installation in a single tunnel because it requires no change to the tube system or the cars.