"True" diners on day LD's is probably not gonna work, for a multitude of reasons. What i think Amtrak needs is something like was posted above, a "Bistro Car".
Staffed by two people, one to take orders and work the Bistro, one to prepare the food and drinks (and other misc). So you say, why two people? If there is only one staffer, then what you offer is severely limited, pretty much what you have on the AmCafes now.
There is a ton ways that two staffers could create kick-ass food offerings, Hell, we did it on the renovated lounge cars that were on the old Broadway Limited when Amtrak was trying to ramp up service, and offer an alternative to the full diner. We had steam tables and all that, but today, one could get by with much easier prep, almost a la Chipotle, or any other Quick Casual establishment.
Perhaps the 2nd staffer would work the coach too, when not at "designated meal times".............. it could happen.......