Some Amtrak trains are reserved while a few operate unreserved. The exact same passenger cars (and hence the accomodations) are used on both (with very rare exception). To confuse the issue even more, the coaches used on most long-distance trains in the Eastern U.S. (ie, those to/from WDW) are virtually identical to the cars utilized on the reserved/unreserved trains in the Northeast (anyone with a knowledge of railroading can point innumerable differences, but the average passenger would probably swear it's the same car). The number of seats per car may differ, but all are light-years above anything seen on a plane or bus.
However, no Amtrak long-distance train is unreserved. You can, though, generally make a reservation even minutes before train departure. Most trains in the Northeast Corridor (Boston to D.C.) are also reserved. Anyone with a valid ticket can travel on one of the unreseved trains along this route, though standees are often a real possibility (not a problem on the reserved trains).
Business class is indeed a added-class accomodation (not worth it, IMHO). First-Class on a long-distance train refers not to coach, but to the sleeper accomodations. Acela Express (different from Acela Regional) does offer a First-Class ticket for a first-class experience (and price) on a third-class trainset (mechanically speaking, anyway). Unless you want the 150-mph Acela experience (fastest train in North America), any "conventional" train will do.