We can do it but it won't be comfortable. It would be "roller coaster fun" (similar acceleration). You have 500 people on a train (half of them facing backwards

We can do it but it won't be comfortable. It would be "roller coaster fun" (similar acceleration). You have 500 people on a train (half of them facing backwards) and want to give them a comfortable ride. Including keeping the coffee in their cups or their stomachs :). Again, it's not a technological challenge, it's a biological one. The human body doesn't evolve faster because we have capable cars. At 5m/s^2 we're talking ~0.5G, more than what you feel in a plane taking off (normally ~0.4G) and one order of magnitude higher than most high-speed trains accelerating (~0.03-0.05G). This is not a normal or comfortable regime. And at 0.2G lateral acceleration (in curves) you have a good chance of seeing your lunch again. This means at 600Km/h you need 15-20Km turn radius. Add to that the air turbulence at close to and over 500Km/h at sea level which causes a rumble that's nausea inducing for most people (riding in low pressure tubes may help here). Just a few other numbers for reference: Shinkansen: 0.07G ICE (German HSR): 0.05G S-Bahn (metro transport): 0.1G Plane take off: 0.4G Roller coaster: 0.5G