We can think of much better things to do with three days in Switzerland. If you've only three days for the country and have a strong desire to see one or the other, spend your time there, not crawling along train tracks. The best parts of the journey are those near and west and east of Chur. Brig-Chur is particularly recommendable since both points are relatively easily accessible from Geneva and Z rich. Moritz segment is probably the more interesting stretch if you don't mind being marooned in St. Moritz.
E ating in the dining car is expensive and reservations are required.
The food is good though, and you'll have a unique experience in the third millennium eating in a genuine antique dining car. Otherwise there's a rolling buffet cart aboard, and a couple of stops leave enough time to dash into the station to grab some take-along food. If you're unable to plan ahead or find reservations unavailable, and you don't want to follow our advice and skip the trip, there is frequent train service along the route without the special designation of 'Glacier Express'. You'll have to change trains a few times, but you'll see the same sights. The full route runs from Chur to Tirano, Italy in something over four hours.