1) I thought it since primary school, so you're kinda slow.
2) Now to realise it's pretty retarded. Metals will be sticking to the cars, parking will be a problem, the repulsion of the magnets may be so strong causes the cars to ramp onto non-magnet (eg: walls) harder.
3)Your last idea is the same concept as bullet train. Since the car is hovering, mind telling how you want to propel forward?