The population density of the NEC is sort of unique in the USA:
Metro area population
New York - 19.6 million'
Washington - 6.2 million
Philadelphia 6.2 million
Boston 4.9 million
Baltimore 2.8 million
Richmond 1.3 million
I don't think any of the other corridors have so many intermediate cities with such large populations.
Plus, more cities in the Northeast all along the line where one can live or visit car-free:
Boston
New York
Philadelphia
Washington
Other parts of the country (e.g., Chicago, San Franscisco. Seattle, Portland) may only have one "car free city" in the corridor. (OK, Seattle and Portland are an exception, but the metros aren't quite as big (Seattle at 4 million and Portland at 2.5 million.)
The NEC also has major regional rail systems feeding into Amtrak in Boston, New York, Newark and Philadelphia, and minor regional rail in Baltimore and Washington. Plus it has a bunch of park and ride stations (New Carrolton, BWI, Metro Park, and Rt. 128) that add suburban ridership that doesn't have access to regional rail.
And for the stuff coming out of Chicago, which does have major regional rail connections, here are the populations:
Chicago 9.4 million
Detroit 4.3 million
Minneapolis/St. Paul 3.7 million
St. Louis 2.8 million
Cincinatti 2.2 million
Kansas City 2.2 million
Indianapolis 2.1 million
Cleveland 2.1 million
Milwaukee 1.5 million
I'm sure they could develop good corridors with hourly plus service in these places outside the NEC, but it will require more than just running trains to make them successful. They'll need to do some heavy-duty transit oriented development around the stations, and [robably build some park and ride stations, too,