Why are maglev trains still rare?
They were invented decades ago.
They have fewer moving parts than wheelbois.
They require less maintenance.
There's obviously some bottleneck in expanding maglev technology, but what is it?
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Sure on a small test track. As soon as it was meant to be scaled up, every attempt has been whittled down. Either it fails completely (Look up Brunel's Atmospheric Railway) or has been so expensive and impractical that it gets reduced to cars in tunnels.
If you are most concerned with efficiency, then building the cheaper HSR infrastructure to get freight off of roads and passengers off of planes as fast as possible should be the first priority. Holding even a partial vacuum in tubes hundreds of miles long just to eke out a little more energy efficiency is laughable. Everything leaks. Maintaining cabin pressure in a 73-meter plane is a completely different beast from maintaining vacuum in miles of tube. It's likely that maintaining the tubes will end up costing so much that any efficiency gains acquired from the vacuum will evaporate.
The vegas loop is its own dumb thing, its not relevant.
For the leak thing, dont just assume whatever wild claim that benefits your side. Heres a DoE study, if you search leak youll find an estimated power usage profile, with large spikes for acceleration, and a sliver of an orange bar across the profile as the baseline load for maintaining vacuum against leaks https://www.energy.gov/eere/vehicles/articles/effect-hyperloop-technologies-electric-grid-and-transportation-energy
When I'm talking about leaks, I'm not talking about the extra energy required to constantly run vacuum pumps. I'm saying that HSR infrastructure needs inspection and occasional repair, but not nearly to the extent that a vacuum tube based solution would. Any savings made via efficiency are pissed away by having to pay more maintenance crews and material cost to maintain the infrastructure. The tubes are also much less likely to be able to be automatically inspected like rails can be using inspection cars because any train moving through the tube can only inspect the interior walls. Besides, rail already exists across much of the US for use as freight infrastructure. These same rails, if inspected and tested properly, can be used for high speed rail much more immediately than waiting for tubes to be built. Besides all of this, more aerodynamic trains can and have been built, but are not in use in the US. Instead, we send bricks down the rails. The "immense" efficiency gain from 0.5 atmospheres of air pressure is likely significantly less impressive when compared against well designed trains with regards to aerodynamics.
All of this is also completely ignoring how dangerous tunnels are for fires. Even with proper safety precautions, fires in tunnels are exceptionally dangerous. By venting out the smoke that kills people, you increase the intensity of the fire that also kills people.
Oh man yeah welded joints are just falling apart every week, it takes a fortune to upkeep some stationary metal.
I dont think you understand air's role in aerodynamics. At half atmosphere...you've plainly removed half the friction, there is half the air molecules to collide with the train. To give some perspective, all of the fastest manmade vehicles, by crazy high magnitudes of difference, are space vehicles. The ISS is whizzing around at nearly 5 miles a second in low earth orbit. Air is the limiting factor in speed. Its the only reason any additional fuel needs burning to maintain speed.
Finally...youre not gonna have a fire in a vacuum tube