My most recent journey was on the Empire Builder in mid-winter, which was amazing. It was dark for about 15 hours a day but there was always something to look at as the train passed through countless little towns decked out in Christmas lights.
Whichever route you choose - and there's something to be said for each of them - make sure that you book a room. Unfortunately, I think the family rooms are on the lower level, with little in the way of a view, but you can book what is called a "bedroom suite", which is two adjacent bedrooms on the top deck with the adjoining door unlocked.
The bedrooms are fantastic, with two full-size, comfortable bunk beds (the lower bunk is big enough for two children), two armchairs either side of a big picture window and a private shower. We both slept very well indeed every night!
The Roomettes are really small and claustrophobic, and not really ideal for a cross-country journey, although they are on the upper deck and have big windows. But you could cut costs by booking a bedroom and a roomette, as the bedrooms are big enough for the entire family during the daytime.
Bookings open about a year ahead of departure, and the sleeping accommodation often sells out quite quickly.
All sleeping accommodation includes three meals a day and unlimited coffee, tea and cold drinks.