It can’t use “air” around it for engines because what’s around it isn’t “air”.
Oxygen is much heavier than the fuel it’s used with, and you’d either need liquid oxygen (which increases costs) or pressurized tanks (which would perhaps double that mass). That’s still lighter than batteries, yes, but engines are also needed. Piston engines are inefficient and/or heavy, and gas turbines are somewhat expensive.
It’s not that difficult to separate water and hydrogen, that’s true, but processing that much gas is still rather impractical when batteries have enough specific energy. Simply condensing it in the tube is...possible, but would increase drag, especially considering density variation issues, and you’d have to deal with getting it out of a long sealed tube without leaking hydrogen.
Also, if batteries are good enough, the cost of replacing the hydrogen alone probably makes batteries better than burning the hydrogen.
Condensation is not just possible but would happen by default. You described the tubes as steel lined with aluminum in contact with the ground, if not buried. That’s going to be consistently cool enough for passive condensation.
Getting water out of a long tube shouldn’t be hard with multiple drains, and if there’s any incline, you just need them at the bottom. You can just dump it in the ground. Use a plumbing trap to keep the gasses separated. They’re at equal pressure, so this should work, and the pressure can also be maintained mostly passively with hydrogen bladders exposed to the atmosphere on the outside, although the burned hydrogen will have to be regenerated before they empty completely, but this can be done anywhere on the pipe. Hydrogen can be easily regenerated by electrolysis of water, which doesn’t seem any more expensive than charging the batteries. It might be even cheaper to crack if off of natural gas or to use white hydrogen when available.
Are turbines more expensive than electric motors for similar power? It’s true that conventional piston engines are heavy, but batteries are also heavy, especially the cheaper chemistries.
Alternatively, run electricity through the pipe to power the vehicles so they don’t have to carry any extra weight for power. It’s coated with conductive aluminum already. If half-pipes could be welded with a dielectric material and not cost any more that would work. Or use an internal monorail, but maybe only if you were going to do that already. Or you could suspend a wire. That’s got to be pretty cheap compared to the pipe itself.
Simpler to do what some existing electric trains do: use the rails as ground, and have a charged third rail for power. We don’t like this system much for new trains, because the third rail is deadly to touch. It’s a bad thing to leave lying on the ground where people can reach it. But in this system, it’s in a tube full of unbreathable hydrogen, so no one is going to casually come across it.
It can’t use “air” around it for engines because what’s around it isn’t “air”.
Oxygen is much heavier than the fuel it’s used with, and you’d either need liquid oxygen (which increases costs) or pressurized tanks (which would perhaps double that mass). That’s still lighter than batteries, yes, but engines are also needed. Piston engines are inefficient and/or heavy, and gas turbines are somewhat expensive.
It’s not that difficult to separate water and hydrogen, that’s true, but processing that much gas is still rather impractical when batteries have enough specific energy. Simply condensing it in the tube is...possible, but would increase drag, especially considering density variation issues, and you’d have to deal with getting it out of a long sealed tube without leaking hydrogen.
Also, if batteries are good enough, the cost of replacing the hydrogen alone probably makes batteries better than burning the hydrogen.
Condensation is not just possible but would happen by default. You described the tubes as steel lined with aluminum in contact with the ground, if not buried. That’s going to be consistently cool enough for passive condensation.
Getting water out of a long tube shouldn’t be hard with multiple drains, and if there’s any incline, you just need them at the bottom. You can just dump it in the ground. Use a plumbing trap to keep the gasses separated. They’re at equal pressure, so this should work, and the pressure can also be maintained mostly passively with hydrogen bladders exposed to the atmosphere on the outside, although the burned hydrogen will have to be regenerated before they empty completely, but this can be done anywhere on the pipe. Hydrogen can be easily regenerated by electrolysis of water, which doesn’t seem any more expensive than charging the batteries. It might be even cheaper to crack if off of natural gas or to use white hydrogen when available.
Are turbines more expensive than electric motors for similar power? It’s true that conventional piston engines are heavy, but batteries are also heavy, especially the cheaper chemistries.
Alternatively, run electricity through the pipe to power the vehicles so they don’t have to carry any extra weight for power. It’s coated with conductive aluminum already. If half-pipes could be welded with a dielectric material and not cost any more that would work. Or use an internal monorail, but maybe only if you were going to do that already. Or you could suspend a wire. That’s got to be pretty cheap compared to the pipe itself.
…run electricity through the pipe…
Simpler to do what some existing electric trains do: use the rails as ground, and have a charged third rail for power. We don’t like this system much for new trains, because the third rail is deadly to touch. It’s a bad thing to leave lying on the ground where people can reach it. But in this system, it’s in a tube full of unbreathable hydrogen, so no one is going to casually come across it.
Using sliding electrical contacts for power is fine for current high-speed trains, but it doesn’t work as well above 200 m/s.