North American narrow gauge railroads essentially had a short history as big time operators so their equipment from the 1870s-80s era tended to be all there was. There were a few exceptions such as Rio Grand closed vestibule cars, the modern cars on the ET&WNC and of course, the Newfoundland Railroad which offered passenger service in sleepers till modern times.

But for real passenger service of the kind we normally envision with the words, standard gauge railroads set the norms. So, before we build a “modern” narrow gauge passenger car, we need to look at the evolution of their standard gauge brethren.   Based on exterior appearance and construction, I think of passenger cars in 6 eras. These eras are of my own concoction and are entirely arbitrary - just the way I tend to think of these cars.

These era are: Early, Maturing Services, Great Wooden Cars, Heavyweights, Streamliners, and Modern.

The era are arbitrary, as there was considerable overlap between them. Seldom did a railroad or builder jump from one era to the other.  New cars might not incorporate the latest innovation.  The chart below shows a timeline of technical innovations: