[jacquesm]

You could conceivably space cars closely enough when they're automated that the aerodynamic drag would be considerably reduced for every car except the front one.

You're still going to have to deal with rolling resistance and going up-hill you'd have to limit the speed of such a convoy to the speed the slowest car can attain.

But for most distances that a car will normally travel the difference between 150 and 300 Km/h will not translate into meaningful changes in time spent on the road and probably you're still going to beat high-speed rail in door-to-door scenarios.