It's the Brand, son.
The idea is bonkers, unless you're hoping for a large slice of government pork. California announces it wants a faster rail link from LA to SF, businesses line up at the trough. As for air..
In Musk's original Hyperloop concept, an electrically driven inlet fan and air compressor would be placed at the nose of the capsule to "actively transfer high-pressure air from the front to the rear of the vessel,"
.. Which is why 'artists impressions' show turbine blades at the rear of the 'pods'. Except being near vacuum, it'll be low pressure air outside the pod. So it'll use rail gu.. I mean linear induction motors for most of the propulsion. Pod people will coast along in comfort, unless the cushion fails. Or there's a kink or snag due to quakes, expansion or sabotage. Implosion, explosion or just stuck. Risk of that might be high given suggestions that the HypeLoop will have a very low operating cost. Which is unlikely if it's routed along the I5 median, or underground.
Then there are other tricky issues like the cost. Not only design and construction, but also per pod. Concept art shows single containers in the cargo pods and small numbers of passengers in the suici.. I mean people pods. So they'll need to be loaded/unloaded safely at costs comparable to existing passenger & freight rates. And the I5 carries a lot of freight. Which is part of the California politics. Roads are falling apart and in dire need of maintenance. A high capacity rail link might take some of the pressure off, or just end up being a very expensive white elephant. If you're spending other people's money, that's not a problem.