All the major metro areas on the Acela corridor are also on a straight line, on significantly flatter land than Japan. Notice how the Acela never spends 10+ minute periods in long, deep tunnels under mountain ranges. The Acela primarily spends most of the trip going below 100 mph because it is operating on 100+ year old infrastructure that has only ever been upgraded piecemeal as it starts to fail.
It's always feels funny to me when taking the Acela between Boston and NYC that you go screaming along at 150mph... for a small portion of track in Rhode Island. The rest of the time you're going much slower. It's almost like, why even bother for that small section?
The Shinkansen was a very different experience when I took it.