The problem with trains in the US is the incompetence of institutions running them.
Acela, for example, was supposed to be a high-speed service, enabled by a tilting mechanism to counter turning forces. But the mechanism is frequently unusable, because the train's tracks are too narrow. It cannot tilt without risk of hitting another train, or a telephone pole. [1] While advertised at 150 mph, it only briefly reaches that speed. Its shorter travel time is mostly a function of being an express. [2]
This was knowable well before Amtrak spent billions on the project, but wasn't realized until after they had. The Acela cars are nice, but their expense was driven by a feature they largely don't use. The whole project is a waste of capital.
What's really needed for high-speed rail between Washington and NY would be straight track. This could be found by going north of DC to Pennsylvania, and then in -- but this would take Baltimore and Philadelphia off the route. A similar route could be found for NY to Boston, but this would abandon Providence. Does anyone think Congress will allow that?
It might also be found by clearing a new right of way by eminent domain. This would already be hugely expensive. And it too would be fraught by politics, as local communities vied to have the route curved to serve them. And can you imagine the corruption possibilities -- to move the route, to gain knowledge of where the route would go for land speculation?
And the construction of the route, and the train equipment, would become opportunities for Congress to take care of their various union and corporate partners, all at tremendous cost, and at risk of the technical requirements of the service.
The US lacks really good rail because it lacks really good civil institutions to build and maintain it. The problem isn't money -- look at the billions that went into Acela -- it is the lack of the competence and focus to actually serve the public interest.
[1] http://articles.courant.com/2000-05-27/news/0005270037_1_til...
[2] http://www.washingtonmonthly.com/features/2001/0110.carr.htm...
See also:
http://www.nytimes.com/2005/04/24/us/acela-built-to-be-rails...
EDIT:
It turns out that Amtrak looked at all this in 2010: http://www.amtrak.com/ccurl/214/393/A-Vision-for-High-Speed-...
Look at the routes they analyze (p. 9 and 11). Rather than look to straighten track, they bend the NY-Boston route to catch such vital places as Westchester Airport, Danbury and Hartford. The DC - NY route turns to catch Baltimore, Wilmington, Philadelphia and Trenton. The economic case studies talk about the benefits of high-speed rail for Hartford and Baltimore. We can only guess at the cost increases and service impairments associated with routing to those cities.
The point of this isn't to get people from DC - NY - Boston faster. It is to get the promise of that service -- which people do want -- to justify enormous spending serving the interests and vanities of incumbent players.