Well, we do have maglev trains such as the ones in Shanghai. I could be wrong but I think the main benefit is the vehicle doesn't need to overcome rolling resistance, making it easier to accelerate and allowing faster travel.
It's lateral and vertical acceleration. You'd have to build rail tracks and roads surprisingly flat and straight to travel across them at above 100 m/s (200mph, 300 km/h). We're talking lateral accelerations of 5-10m/s², or above 0.5g. This can be achieved with very well maintained tracks only, which is a second limit to high speeds besides high power consumption. There was some discussion about this when Elon Musk published his railway ideas.
