Underwater tubes have some other issues, like an extra atmosphere of external pressure for every 33 feet of depth. Neutral buoyancy seems like a good idea but it still requires rigid anchoring (sensitivity to earthquakes AND wave action) to handle the occasion of a ‘car’ stopping in case of emergency. Unless you envision a tube so flexible it can just drop from the extra mass of the stopped car.
It is also an extra layer of hostile environment in the case of a bail-out. If they are above ground the car might have a means of popping a tube ‘hatch’ into open air to escape. Admittedly the tube’s vacuum is an even more human-hostile environment than under water.
