A bit of an answer
Mark either doesn't quite understand what happens in the creation of liquid nitrogen or explain it badly. But what happens in the creation of liquid nitrogen is also part of the answer to your question about "fireless locomotives".
The boiling temperature of any liquid is directly proportional to the temperature but inversely proportional to the pressure. This creates some interesting phenomena. It's why you can use steam to cool a building. If we take water, inject it into a partial-vacuum such that the water's temperature is above the boiling point for that particular pressure, the H2O boils, and in the process, absorbs heat. When we then return the H2O to "room temperature and pressure" it condenses, thus releasing its absorbed heat. This is known as steam refrigeration.
Fireless steam locomotives place the water in a closed container, and then super-heat it. By reducing the pressure within the container, the water is allowed to boil, thus turning it into steam. The problem is, you're dealing with a super-heated material under high pressure.
Using liquid nitrogen means compressing the air. But as you compress the air, it loses its capacity to store heat and thus releases heat. So you carry off the heat to use somewhere else. Once the nitrogen is compressed until it is a liquid, and the excess heat has been removed, you then run into the problem of storing it since if it absorbs heat, it will boil away. Luckily, Liquid Nitrogen is fairly easy to store within a vacuum-walled thermos bottle without it being under high pressure. So while you use high pressure to make it, you don't need high pressure to store it. This makes it a safer storage medium.
