I don't care much for buffet (the "take-all-you-want", type) restaurants on land; and like the idea even less on a moving train...
First of all, there is gross waste of food. Secondly, elderly passengers would have a difficult time carrying trays on a fast-moving train, especially over rough areas of tracks, although waiter assistance could solve that problem....
Not to mention the unsanitary actions of some children and even some adults in handling foods....(can you say norovirus outbreak?).
As for domes being a "unique benefit that only rail travel can provide"....the old Greyhound "Scenicruiser" (GM PD-4501), built in the mid fifties, sure came close....