In my experience over the years, you get the following on the Cap for dinner:
-Chicken (guaranteed on most trains as a listed menu item)
-Vegetarian pasta of some kind (usually to be avoided, as it is the most easily bungled and is more or less an ersatz fill-in to satisfy vegetarians)
-Steak (of some kind; guaranteed, though the cut will vary; NY Strip is very popular, and Amtrak has good steak buyers)
Other than this, you have a variable seafood dish. Types of fish are popular, but the Cap also often has crabcakes (which were always a winner for me). And finally, there is a "mystery dish" (the daily special), which can vary from braised beef (good when I had it on the SWC) to another seafood dish to just about anything else. At the holidays, there is frequently an additional turkey dinner available in addition to the regular menu items (turkey, stuffing, etc.); it tends to be solid but not spectacular, but I'll buy it both to get a variation from the steak (which I buy about 2/3 of the time) and because of the season.
As to dessert, you are guaranteed a "chocolate" option and a "cheesecake" option, as well as ice cream and/or sorbet on the menu. This can vary widely...popular desserts have included creme brulee cheesecake and peanut butter pie (both from the Meteor, both excellent). Other than this, the quantity and variety of selections can vary wildly from train to train.
With breakfast: French toast and bacon is my preference. The scrambled eggs are a roll of the dice, and I've never had either the quiche (the usual "good morning special" is a quiche of some kind) or the continental (though I did get someone else's grapefruit when they declined theirs due to health concerns).
Finally, as a caveat: Anything can sell out, and the last seating can get a little "tight" on this front.