I agree with most of your comment except for this one:

> Air travel is not only the fastest form of travel in common use, it's also one of the most efficient, due to the thin air at cruising altitudes

While airplanes are slightly more efficient than cars at transporting passengers on a distance/energy basis (and only if you exclude electric cars), trains are still much more efficient by about 30%. See https://en.wikipedia.org/wiki/Energy_efficiency_in_transport...

With respect to carrying freight, cargo ships are the most efficient of all.

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dlcarrier 5 hours ago | parent [-]

I'm talking about the physics, not the logistics. Logistics are a present challenge, but physics are a hard limit, so load capacity and power conversion losses don't really have the same insurmountable bearing as rolling resistance and drag. Drag increases with the square of the speed, so fuel efficiency is really only comparable at a given speed.

Water is extremely viscous, so it creates a lot of drag, and cargo ships must scale to huge sizes and stay under 20 knots to have high efficiency. Trains are only flowing through air, which is much thinner than water, so freight trains and most passenger rail go double or triple that speed. High-speed rail doubles to triples the speed again, but is much, much less efficient than rail traveling at freight speeds, because it has no means of avoiding the added drag.

If you were to try and increase high-speed rail by another two to three times, you'd need a rarefied tunnel to make it approach being practical, which is basically what Hyperloop proposed. It's much more practical to use the already existing rarefied atmosphere at high altitudes, which is why airplanes have unmatched efficiency for travel at those speeds.

When travel time going into days or even weeks doesn't matter, e.g. for cargo, all that matters to efficiency is how slow you can go. When time is taken into consideration, as speeds go up rail becomes more efficient than boats, but they top out and airplanes are more efficient than rail. Eventually sub-orbital rockets become more efficient than airplanes, but there isn't a lot of travel needed at those speeds.

	▲	stephen_g 3 hours ago \| parent [-]
		That’s ignoring a lot of other losses. Like in air travel you’re instantly losing 10-15% of your fuel generating lift, then you have to add in the fuel used for carrying the other fuel required throughout the flight. Whereas with high speed rail, rolling resistance (which we could say is the equivalent of lift in aircraft) is less than 1% from a quick search, and for a lot of passenger rail and almost all high speed rail you don’t need to carry fuel which is supplied as electricity from an overhead line. Hyperloop of course was always a non-starter because the kind of energy that would be required to pump down such a huge volume would be ludicrous, maintaining the low pressure extremely difficult, and the amount of materials needed for the tube would be ridiculously large.