[saturn]

> There is less air friction if your tunnel is maintained at vacuum.

Indeed there is zero! Realistically speaking of course, a hard vacuum will be difficult. Even a 0.10 atmosphere would be a huge improvement. Or the intriguing possibility of pressurising but with a less dense gas. Helium, for example is about 1/10th the usual mix of gasses on earth. You could put a plastic sleeve over the shinkansen routes, fill it with helium, and see air resistance drop 90%. We might not actually have enough helium for that, though.

I'm just trying to say, there are options, if we truly care about speeding up point to point travel in-atmosphere.