The Amtrak trains between Newark and Orlando/Kissimmee (and on to Miami) use Amfleet II coaches (seat 59 each), very similar to the Amfleet I/Metroliner cars used north and south of Newark to Boston and Washington. There's more legroom (fewer seats, all reclining), legrests in every car, larger windows, only one vestibule, and a few other minor differences in Amfleet II (built for long distance travel, while most of the I series cars were not - at least originally). There are also two Viewliner sleepers on the train, a lounge car and dining car serving full meals (and generally very good meals at that).
The seats are reasonably comfortable for spending one night (gets a little old going all the way to
Disneyland in California!). You may want to have something to read or activities for any children; After dark there isn't much scenery to see out the window. Incidentally, if you want the best choice of seats (such as mid-car, close or far from restrooms, etc) you might board at Penn Station in NY (individual seats are not assigned).
My father asked the conductor and was told that the Amtrak train using tracks that are also used by freight trains, and that the freight trains always have the right of way, thus the delays to allow the freight trains to pass or cross the tracks.
Actually, by law Amtrak always has the right of way over freight trains. However, this "law" is broken all the time - about as often as the speed limit in the interstate! Dispatchers - who work for the freight railroad who owns the tracks - sometimes give their own freight trains priority over the passenger trains (by some of their own estimates, the amount Amtrak pays them to handle the passenger trains doesn't cover the cost of doing so, so it's easy to understand why they would give a green signal to one of their own operations first). This isn't right, and it's not supposed to work like that - but often does. Delays are so often traceable to freight train "interference". Still, the Florida trains have an excellent on-time arrival performance, and delays - when the occur - are usually not very long. Also, it's sometimes possible to make up lost time en route. I wouldn't worry about it at all (last July on this route my train was 45 minutes early).
The other side to this story is passenger train interference to freight traffic. The faster Amtrak train overtakes slower freights, who have to stop in a siding and wait for it to pass. A 125 car train just sitting for an hour or so costs money! Hence, the delays continue.
I think you will enjoy the train; it really is a great way to travel. I tend to think of my "magical" vacation experience starting when I board the train, not when I arrive in Florida. Have a wonderful trip.