You don't build it for Boston to Chicago, you build it for Boston to Detroit and Chicago to Toronto -- essentially the same thing you've described about NEC. It's not the end to end it's the missing segment overlap that is the issue.
Who are these "you" that (can afford to) build networks that way or have an established track record at this?
Japan is a freakish outlier, having 127m people in an area the size of California (pop. 37m) or our NEC (pop. 55m).
Please, I beg, you, understand the USA map and the FRA map both as having essentially 3 cities that are worthy of Euro-style service NYC (pop 8m to 17m), CHI (pop. 3m to 10m) and LA--(pop 4m to 13m)
The reality in places in Europe that are analogous to the USA you get spiders centered on mega-cities and national capitals, and those spiders don't touch at the ends, and won't for 40 years after the initiation of service.
Spain's is centered on Madrid (pop 5m) (
http://en.wikipedia.org/wiki/File:AVE.png)
Italy's is centered on Rome (pop. 3m) (
http://en.wikipedia.org/wiki/File:Italy_TAV.png)
France is centered on Paris (pop 11m) (see below)
And the UK basically has only (recently) done their HSR line from the Chunnel to London (before that it was conventional rail, just like BUF-CLE on the FRA map)
And in Paris many of the trains don't even run through. TGV service started in 1981, and is still basically a spider diagram, and it will be 40 years from start of service before it gets connected to Spain (2020) or Italy (in 2023) or goes beyond London (2026 on HS2).
Here is a great map showing the shape and chronological roll out of the TGV in France. Note the number of "missing" connections:
Perpignan to Montpelier (which would connect the Spanish AVE to the French TGV)
Bayonee to Bordeau (which would be the other way to connect Spain to France)
Lyon to Turin (which won't happen to 2023)
Strasbourg (which won't happen to 2016...35 years after TGV's first)
Also note that even though Paris-Limoges-Toulouse is a long-established overnight train route, they are not upgrading it *ever*. Paris-Toulouse is analogous to CHI-Toledo. Better to serve each of Limoge and Toulouse as branches than build a whole parallel line straight north-south. Similarly, serving Toledo makes more sense as a branch from Detroit.