We decided to take the luxury dome car (a bit pricey) because our parents said it was worth it. It was just redone via a $1 M investment and you could tell. No children under 15 are allowed in the luxury dome. The seats were assigned, which we liked, because it eliminated the 'sparring for spots' sport that many people seem to enjoy.
The train attendants were very good natured and very accommodating. They seemed genuinely enthusiastic about making sure everyone enjoyed the trip. The food was fresh and delicious. Alcoholic drinks were available for purchase. Soda, coffee, tea and water were all provided. The facilities were clean and well kept.
Note that you can travel a different class each way so could potentially save some $ if you wanted to go luxury one way and coach another. I would suggest the luxury dome on the return to Williams, because you get a champagne toast and the train robbers and Marshall will spend time with you. Musicians on board also provide entertainment. It's great to sit back and relax after a day of walking/hiking.
In the luxury dome, you can step out onto the back platform and enjoy some fresh air. The train travels about 45 mi/hour so it does take a while to get to the Grand Canyon. You do not see the Grand Canyon at any point while you are on the train, which is something I really wanted to know before we purchased.
The convenience factor, however, was very nice. The train drops you at the South Rim 'hub' and, if you add a bus tour, they are waiting for you just steps away.
We added a short bus tour to our trip so we would be sure to see the primary vista points; totally worth it. The first time you approach the Grand Canyon, it will literally take your breath away! You don't realize how photos cannot do it justice until you step to the edge.
If you travel with children, keep a good eye and grip on them. It's a mile down and the railings do not go the entire length of the viewing areas and are non-existent in other areas. They will tell you it is a wilderness area and that you have to watch your step. (Two days after we were there, two people were swept up in one of the creeks (very rapid water) at the bottom of the Canyon) and along the rim, another woman fell into a crevice; they were able to rescue her.)
Do make sure you head for the train on time. They will warn you that they will leave with or without you. There is only one trip per day. And your ticket is valid only for your designated date.
Hope this helps! Let me know if you have questions! Happy and Safe travels to you!