The invention of the word "heritage" seems to make people think that there is one specific kind of car called Heritage. That is not. It is an arbitrary name Amtrak coined to designate those old cars it wanted to keep and to have Head End Power.
It is just that Amtrak whittled down its inherited sleeping cars to use the type which had 10 roomettes and 6 double bedrooms. This makes people think that all cars before Amtrak were of this configuration.
Not so. There were eight or nine different kinds of rooms in the old cars with many different floor plans. Keep in mind that there were many railroads operating many passenger trains in the past. So do not expect the kind of uniformity on a national scale that we have today on Amtrak. The whole world of sleepers, diners, lounges,etc was quite different from today. Much diversity.
The old cars which VIA kept had, when built in 1954, four sections, four roomettes, five double bedrooms and one compartment. They have been reworked to have three sections, four roomettes and six double bedrooms, though they call them by other names. There is another floor plan on the Canadian but not sure from memory what it is, and cannot look it up because I am on my way out to dinner.
Comfort is a different thing to different people but I, old enough to remember the old days, do not think there was much difference.
All the major builders participated. Budd, American Car and Foundry and Pullman Standard. The VIA equipment is all or most Budd. Budd was the best.
One biggie that Amtrak has over the past is the use of showers. The old trains had very few showers. It was mostly just one room in one car of one train that had a shower, and that room was normally known as a Master Room.
Another is the coffee, orange juice, etc the old trains did not have that in the sleepers.
Many trains were made of stainless steel. The word usage you note is an advertising ploy, not an exact designation. All of the Canadian's equipment is stainless steel. That was a big deal when stainless steel was new.
Thee was no such things as accessible rooms in the past. all of society has improved in those ways.