has there ever been a couchette equivalent on Amtrak? I could imagine that lot of travellers (esp. European backpackers, etc...->big part of passengers on overnight trains!) would love it, if there are economy non-private sleepers (couchettes) that were available for a "hostel night" supplement like $20-30 per night.

There is no real alternative between a seat which is not that much more comfortable than greyhound and the private Sleeper which is far from being affordable to many people. As a couchette car can accomodate as many people as a normal seating car, I believe it would also be a good deal for Amtrak to introduce such an offer.