I gotta agree with Dax, the meals on Amtrak CAN be good, but are usually IMHO, just "average" at best. While you might get a nice cut of steak, on the same plate the potato might be hard as a rock, and you can almost always count on the veggies being cooked to death.
While one usually does get offered a wide variety, including vegetarian, the gourmand among us usually don't write home about the meals on Amtrak.
Now, having said that, one will find that there are indeed many, many "fans" of the meals that Amtrak puts out in their full service dining cars. Why the wide range of opinion? Well obviously different people have different tastes and standards, etc., etc.
However, I think in general you will find that younger travelers (I'm guessing here, maybe under 35) will be much more appreciative of the meals served in the Amtrak dining cars, because they have only experienced "airplane food" or Amcafe offerings before while traveling via rail.
Old farts like me (52) are less than satisfied, partly because we know what COULD be served, and what WAS served on the trains, both in the early years of Amtrak, and of course without question, the "glory days of railroad dining".
As a dining car employee in the late 70's and early 80's, I saw the Amtrak chefs push out some pretty amazing meals, with pride. When Congress pushed this "modified meal service" crap onto Amtrak, and they cut back both the staff, and meal options, it was downhill from there.
They are a lot of Amtrak chefs who do amazing things today, given what they have to work with, but you can only do so much, with what you have.
As many here will note, I have often stated that one can still have a "great dining experience" even if the food is not four-star. The friends, atmosphere, and the SERVICE ("It's The Service Stoopid!") can all make an average-tasting meal be overlooked. (a bottle of wine doesn't hurt either!)
So my summary is this: Don't expect cruise-style or five-star meals, you won't get it. But you may be very pleasantly surprised from time-to-time. If you keep your expectations in check, and don't let some minor transgression bug you. (like serving very good tasting sorbet after dinner, but serving it in the original little cardboard cup it comes in, delivered to you in a plastic bowl........Grrrrrrrrrr) then you will be fine.