But beyond air drag is the force required to move the weight of the train, and overcome the friction of the wheels. Long freight trains require several locomotives — much more power — to move all that weight. The benefits of the hyperloop pod (if handled right ) is the physics of running in a near vacuum, rather than through air, and a suspension system that becomes almost frictionless at speed, allowing the pod to coast long distances at high velocity with only an occasional “push” — meaning much less energy is required.
I agree with most of what you are saying, but with a couple of side notes :
Air friction (and thus energy usage) increases with the square of speed of the train.
So if you want to go twice the speed as HSR, you increase energy use due to air friction by a factor of 4. Hyperloop wants to mitigate that by reducing the air density, but this goes only linearly. So if you want to go twice the speed, and keep air friction the same, you have to reduce air density by a factor of 4 (go to 250 mb).
The problem is that creating lower pressure also demands energy. Especially when compensating for leaks. The NASA paper that Sleepy posted previously :
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170001624.pdfgives a good overview of when it makes sense to put the train in a tube, and which pressure it should operate on to be more energy efficient than simply building a HSR system. See fig. 12.
This suggests that yes, it makes sense to lower the pressure in the tube, but it may not be energy efficient to operate it at the near-vacuum conditions, and ultra-high speeds, that Elon originally anticipated.
And then there is the issue of the pod cross area versus the tube cross area, which puts essentially an upper limit on the speed you can run the pod at (at least without resorting to exotic new technologies like putting a large vacuum pump in front of the pod running at 10X the RPMs of a jet engine).
My guess as an engineer is that the first real commercial Hyperloop (if it would be built at all) would be a traditional MagLev running at speeds less than 300 mph inside a tube at reduced pressure but not near vacuum.