First, 300 mph (480 kmh) is not on the cards. It's feasible technically but the energy usage rises exponentially, so it's quickly less attractive. (Obviously planes are even more inefficient for exactly the same reason, but wind resistance is at least reduced at altitude.) This is why speeds tend to top out at around 300 kmh, with only a very few prestige services (in China mainly) going marginally faster.

The good news is that 300 kmh is really fast, and rail is usually much more time-efficient at the endpoints (i.e. stations are more central and there's less waiting). In China, Shanghai-Beijing (approximately LA to San Francisco) is now possible in 4h30. Given the endpoint advantage, that is more than competitive with plane travel, and much more pleasant.

As of this year, China also has a high-speed capsule sleeper that gets you from Beijing to Guangzhou (CORRECTION: 1875 km) in 12 hours at an average speed of 180 kmh. It's still night travel, not every American would be interested. But it shows what's possible.