First, Japan doesn't have any (operational) trains going 250mph. The Tokaido shinkansen between Tokyo and Osaka has a top speed of 300kph, which is 186mph. The Tohoku line in north Japan reaches 320kph (200mph) according to Wikipedia, and this seems to be the fastest in the country (I haven't ridden that one myself, but I have ridden the Tokaido multipel times and verified its speed with a GPS speedometer app.). The new maglev Chuo shinkansen, when it's open for service in 10 years, will go 505kph (314mph). China does have some maglev train currently; I can't be bothered to look up its max speed, but it's a short line that just goes to that city's airport. It's not a long-distance train line, really more of a demo.
America has at least 2 trains that go over 100mph: the Acela, which runs from DC to Boston (not NYC), and the Northeast Regional, which runs on the same route on the same tracks. The Northeast Regional hits a max speed of 125mph, and the Acela hits 150mph. The catch is, these trains only hit their max speeds for somewhat brief durations on certain parts of the line (mainly between Philly and Newark NJ), so their average speeds are actually rather lousy because in other places they have to slow down a lot because the track sucks, the tunnels are a century old, there's a cargo train in the way, etc. If the US built an all-new track (with new tunnels where needed) between DC and Boston, they could use the exact same trains they use now and have much, much faster average speeds, but they really don't want to do that.
Finally, there is a definition in the US for "high speed rail", and the Acela meets that definition because its max speed is pretty high. (Of course, the definition completely ignores average speed.)