This map shows Amtrak stations by ridership; the bigger the circles, the more riders the station serves. Rail usage is dominated by the New York to DC route, which connects the two highest-ridership stations and has #3 Philadelphia in between them. Those three cities accounted for over 18 million boardings and alightings in 2013 (about 30 percent of total boardings). The eighth (Baltimore), 11th (Wilmington), 12th (BWI), and 15th (Newark) most popular stations are also on this corridor, adding another 3 million passengers or so.
Of course some of the ridership from the NYC-Philly-DC stations is bound for New Haven, Providence, Boston and other cities on the northern leg of the Northeast Corridor. But outside of the Boston-Washington Megalopolis, there's very little ridership happening, and even within it the bulk of the riders are in the southern half.
The vast majority of the stations and routes have very few riders, and you can see these absurd strings of little-used stations dotting the southeast and midwest. Providing that geographically expansive service is smart politics for a federally-run agency that needs to maximize its base of support, but it makes very little business or transportation sense.