We did this as a side trip off our Route 66 roadtrip and stayed at the Grand Canyon Railway Hotel as part of the package.
The day starts with a cowboy "show" before everyone boards the train. We opted for the first class carriage which was extremely comfortable and had loads of legroom.
Tina, the host, provided commentary and information on the destination as well as telling some of the worst jokes I've heard in a long time! She was funny though. There was complimentary pastries, juice and drinks on the journey up and cheese, cold meats and soft drinks provided on the journey back down. You could also have paid-for alcoholic drinks served at the seat if you wanted.
Entertainment both ways on the journey was in the form of a couple of singers who were entertaining and could hold a tune. And on the return journey there was a mock holdup which was fun.
It's a slow train (max 40mph) and takes around 2.5 hrs each way by the time you've boarded etc. and you get about three hours at the Grand Canyon itself. You have the option of a paid-for bus tour when you get there but we didn't bother with that.
My only criticism would be the fact that, as others have mentioned here, you're made to feel a little obliged to tip every step of the way; first the singers, then the cowboys and finally the host who, as we approached home, put on a pair of white gloves and worked her way through the carriage shaking everyone's hand... maybe it's cos I'm British but I'd rather have left a tip unprompted on the way out.
BUT, don't let that put you off - it's a great day out and a fantastic way to get to the Grand Canyon.
Tip: If you're over 50, join the AARP for a modest fee to get a pretty good discount on the hotel and train ticket price, even if you're from outside the USA!