"supersonic" relative to the sound speed at normal density I assume. With no air, there would be no sound!
Edit: corrected pressure to density
Edit: corrected pressure to density
BTW, speed of sound doesn't change (to first order) with changing pressure (edit: nor does it change (to first order) with density, assuming temperature is the same). Mostly effected by temperature.
Hyperloop uses a partial vacuum, of course. No true vacuum exists.
How high is density of atoms in space? Could there be a 1mm x 1mm space without atoms?
"Plasma with a number density of less than one hydrogen atom per cubic metre and a temperature of millions of kelvins in the space between galaxies accounts for most of the baryonic (ordinary) matter in outer space; local concentrations have condensed into stars and galaxies." - https://en.wikipedia.org/wiki/Outer_space
I knew that intergalactic space could see densities of one atom per some human-relevant volume of space, but I had no idea that actually accounted for most of what there is!