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,

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.