Also, "double deck" type of equipment by whatever name, ie. Superliner today, hi-level or "dome" in the past--has often been associated with scenery as well as tunnel clearances or lack of same.
Pre-Amtrak, most of the trains with domes went from Chicago to various West Coast destinations.
Sure, thre were some others, such as St. Louis to Texas points, Washington to Chicago, Chicago to Miami, Richmond to Miami-----but most domes and later hi-level (on El Capitan, from CHi to LA) were longer distance "out west" routes.
Suh developments both referred to what was usually considered to be more spectacular scenery, (i.e. mountains, deserts, etc) but the unrestricted tunnel clearnances also contributed to domes or other such "double deck" being largely on those routes.
Of course there is a completely unrelated double deck design in commuter trains but that was about piling as many people on board as possible,not about scenery!!