I don't think it has much to do with air density as it does with guideway inductivity issues - which would limit it's top speed (aside from the obvious things l

I don't think it has much to do with air density as it does with guideway inductivity issues - which would limit it's top speed (aside from the obvious things like how curved the path is, if and where it has to make more gradual stops and turns, etc). Jet propulsion and magnetic propulsion are fundamentally different. There are limits to what those magnets can push/pull, how much energy can be put into the system, and the stresses involved coming from the fields. http://en.wikipedia.org/wiki/Electromagnetic_propulsion http://en.wikipedia.org/wiki/Electromagnetic_propulsion