By "cars" do you mean the original idea of the public's cars or the company-owned taxis it has evolved/devolved to?
The history of the Loop (the Las Vegas type) started with Musk being frustrated in a traffic jam on his commute. He wished he had a tunnel to overtake the jam (we are supposed to think that makes him a genius). He would descend
sitting in his car to the tunnel in a large lift and ascend again once ahead of everyone else, possibly in his office car park. Moreover, on the lifts and inside the tunnel his car would be on a high speed car carrier he calls a sled (I think he talked about 150mph). The idea is shown in the artist's impession below.
I said "his" car and not yours and mine because the maximum throughput in cars/people per hour would be abysmal; the car lifts would be a severe bottleneck. It would only work if there were few users, presumably Musk himself and some other wealthy subsribers. It would be like any other rat run - only advantageous if few use it. Even Musk realised this would be politically unacceptable so he added the idea of dedicated mini-buses to carry pedestrians. These are the 12 or 16-seater vehicles. Of course he could have proposed cars entering the tunnels by ramps, just like countless existing vehicle tunnels, but this would have been too commonplace and even his fans might have find it hard to claim he had invented anything, and would also have made the sleds a bit pointless.
The LV Loop as built, a small bore ordinary tunnel with a taxi service, with the underground stations reached by escalator, is a massive and humiliating downgrade from Musk's original idea. These days they don't like you seeing pictures like that below. Note that the sleds in the artist's impessions would be too large to fit in the LV tunnels as built anyway, which are only just large enough for a car alone.
There was one publicity run in Virgin Hyperloop's 500m straight and level test tube in a desert; later it was announced that the plan to carry passengers was abandoned in favour of freight only. The real thing that would kill Hyperloop even if the technical problems were solved is that the tube must be almost straight in both the vertical and horizontal planes, otherwise the centrifugal forces on passengers and even freight at the proposed speed would be intolerable. Try drawing a near straight line between two major cities and see what it cuts through. Vertically, it would mean extremely high viaducts and far longer tunnels than even a high speed railway would need.
Financially, few people seem to realise that Musk's calculations assume that because the tube is raised on pylons (at least in flat country) he would not need to pay for the land below. He also neglects to include the costs of signalling. Reading Musk's Hyperloop "White Paper" it is clear that not only does he hate railways with a vengance, he has no awareness of the need for signalling or other control on a system like this. If anything, he seems to assume control by departure interval.
Such a system has no relationship to Hyperloop.
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