I did it nearly 2 years ago, and I doubt I ever will again.
This was in August, and PPs are correct, it did get chilly at night. But near the end of our trip, we had a lot of slow downs, and when the train was barely moving (or stopped) we had no AC. Not fun.
The dining car was crappy gas station food at Disney world prices. I packed our own lunch for day 1, and breakfast for day 2, so we only had to suffer through microwaved pizza for dinner. We did have real food in the dining car on the way back, and that was really expensive but half decent.
The stops.... they weren't really stops, more like pauses. My trip was supposed to have a 90 minute stop in DC, but we were very late leaving NY (there was a hurricane headed toward Tampa, they were thinking of cancelling the whole run) so we had a 20 minute stop in DC. The rest of the stops were 5-10 minutes, just long enough for people to come back in stinking the place up with their smoke-smell. They also decided not to travel past Orlando, so get off and find your own way from here everyone!
The staff were great, all of them. Very nice to my kids, helpful whenever they could be.
The seats weren't bad. Had the fold-down tray table, worked well for food and for my laptop. It is connected more to the floor than the seat, so even my wiggly kids didn't annoy the person in front of them by leaning/bouncing on it. There is also a fold-down foot rest at each seat, and it adjusts to be the right height for you. I really have no complaints about the seats. My kids made a nice little nest bed in their 2 seats, slept pretty good considering. Oh, and there were outlets at each row, very helpful.
You will also need to be willing to put up with other, possibly stupid, human beings for 24+ hours, with no escape. IE: the young parents headed north. Dad starts drinking as soon as he gets on the train. Mom prefers her phone to her kid. Puts the baby to sleep at 3pm, doesn't bother to wake her up. Baby wakes up at 8pm, no way will that baby be getting back to sleep tonight. 1am: baby is crying, mom has been playing Dora (very loudly) on her phone, now proceeds to walk in and out of the train car rocking baby, through the very loud door, over and over again as though she hopes Dad will wake from his drunken slumber and help her quiet baby. Ha.
My problem, and the reason I won't do it again - we were an odd number. My kids got seats next to each other, and had a blast. I got a strange man beside me each way (south stranger than north) and it's very uncomfortable to sleep next to a strange man. If I had even numbers, I might consider it.
My advice to you:
Bring food (that doesn't need to be hot or cold).
Bring activities, just like you would do in the car.
Bring a blanket, wear layers.
EAR PLUGS!
Pack your sense of adventure.
Keep your valuables out of sight, but near you at all times.