How high is the remaining air pressure, as full vacuum is not achievable by theory and known technologies? Would the remaining air constitute some resistance, as the capsule travels faster than sound?

The optimum vacuum level produces the minimum total energy use.  The energy is minimized when pumping power is equal to propulsion power.  Higher quality vacuum reduces drag, but takes more energy to produce.  Optimum vacuum is a function of many variables including: design speed, material leak rates (tube wall, capsule wall, and seals), airlock cycle rate and undisplaced volume, number of capsules per unit of time, etc.  Use of a medium grade vacuum of 10ee-3 Torr, to 10ee-5 Torr is our target for our 600km/h target design speed.  This level can be achieved with a single stage pumping, but two stage is more efficient.  A old CRT style TV is evacuated to a thousand times higher quality vacuum. 

At the target range, the drag will be reduced by a factor of about one million.  Higher speeds and more intense traffic favor deeper vacuum.  There is some drag, but the coasting distance is very long, and speed loss is low.  It is estimated that at 4kmph the drag energy loss for a 8k mile trip is about 8kWh (out of a total kinetic energy of about 250kWh).   Up to 10ee-9 Torr is achieved for very large enclosures such as NASA space simulation buildings, and the LIGO observatory.  Space is about 10ee-12 Torr up to 10ee-19 Torr.  The record vacuum level is better than 10ee-13 Torr.

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