And, while Japanese rail service in general is pretty much as good as anyplace in the world, the Shinkansen is not inexpensive. It's nonetheless popular because HSR is a good match for the distances between a number of populous cities.
There's no reason to think that what makes train service in Japan better than in America is related to it being privatized.
I have total faith that we could privatize Amtrak and have it be just as execrable in comparison. And then it would be sold off for parts, some assholes would get tremendously wealthy, and we wouldn't have any passenger rail network at all.