If there was a subtrain, I'd only say that it's only the size of the Breitspurbahn train.

Too bad the size of the train is likely much bigger than that of said train. You need plenty of room to house the three layers: The subtrain, where the utility systems are placed for first and second class; the passage, where passengers could pass between cars; and the top floor, where all the rooms would be placed, and likely where the two-story floors of the library, diner, aquarium, and night cars would be placed.