Excellent! Ryan's post was at the bottom of the page on my browser and got overlooked. Thanx, Ryan.I haven't found anything on Amtrak's website detailing it.
That being said, it's in the Amtrak Service Standards Manual (retrieved from Amtrak through a FOIA request and screenshot above by Ryan, although I think his is newer than what I have through a FOIA request) and some people here, including myself, have booked them before and used them without issue. They're not often used, so agents don't know much about them, but they do exist even if they're not listed directly on the Amtrak's website.