This article claims that Elon Musk's Hyperloop tunnel is nothing more than a paved underground tunnel that provides dedicated right-of-way for vehicles driving through it.
That's not quite accurate, though.
A tunnel's ventilation shafts can be controlled to control the direction and speed of air-flow, especially if there are fans used to power the air-flow.
So, as with the original Hyperloop design, solar power above ground could power fans the pump air in and out of the tunnel at various points to reduce wind-drag and even increase air-pressure behind a vehicle, so that it would be pushed like a sail-boat.
Of course, to get the most out of the air-pressure 'push,' the vehicle would need to conform more closely to the shape of the tunnel; or at least have a sail that does.
Given air-pressure assist from smart-ventilation of the tunnel, vehicles traveling through it could reach very high speeds.
It would be like driving with a hurricane as tail-wind.