Dennis,
I haven't seen coupons used in quite sometime. Maybe doing away with this was a small way for Amtrak to cut cost.
On my recent trips except for the Empire Builder, it almost seemed like it was they "took your word for it". Meaning we just wrote our name, car and room number. Nobody checked anything. I think this only works when you walk into the Diner from the sleeping car end, and not the end toward the coaches.
On the Empire Builder last month they did check our ticket stubs. However this train has sleepers on the very front and the very rear of the train. So they can't assume your in a sleeper just from which door you walked in. Or maybe it all depends on who is running the show on each train.