When it comes to rail in the U.S., there’s basically two places: The Boston-New York-Philadelphia-Baltimore-Washington corridor, and the rest of the country.
Rail used to be the main way to get between cities in the U.S., but the Interstate offered faster and more convenient travel for those with a car, and really long distances like going from one coast to the other can only be done quickly by air. (For instance, even the fastest high-speed trains wouldn’t be competitive for the New York to Los Angeles route.)
Amtrak, the national rail company, doesn’t own its tracks in most of the country, so its trains often have to wait for freight trains to go first. Coverage and routes are limited. If you live in Memphis, you can go to Chicago fine, but getting to St. Louis might take days even though it’s only four hours by car.
For the big cities between Boston and Washington, rail is still ideal. Traffic is bad, many people don’t use cars, and airports are far from the city center. Traditional rail still does pretty well, and high-speed rail would be incredible. There’s still two problems. One is crash-test regulations, which force trains to be heavier and slower than foreign counterparts. (Other countries don’t have such strict regulations; rail collisions are quite rare, and traveling by train is much safer than by automobile anyway.) The other is simply the cost of building rail lines in that area which can handle high-speed rail. Amtrak’s proposal for true high-speed rail in that corridor is projected to cost over $100 billion.