@GG1 4877 posted:That is a great question and I really don't have a good answer for it. I have always referred to that era as Amtrak's transition era, or the steam heated era. I will need to do more research on that topic. I suppose technically they are all heritage cars, but the Heritage Fleet is a specific program of rebuilding.
One resource for cars that ended up in the Heritage Fleet is this wiki page.
I second calling it steam heat era or transition era. Describes it perfectly