I always travel alone and in a roomette whenever I'm on an overnight train...the exception being 66/67, which may someday get its sleeper back.
As far as dining car tablemates, Experience has shown it's hit and miss as to what passengers I get seated with. As an early riser for breakfast, it's first-come-first-served for seating, as directed by the LSA. Many times there's only 1 or 2 tablemates at that hour, and may be either coach or sleeping car passengers. I've even had breakfast alone several times. At lunch and dinner, there's usually a sufficient number of sleeper passengers showing up at the same time that I'm usually seated with a couple, sometimes with one child. At other times, there'll be a couple and another single sleeping car traveler with me. Depending on the train and/or LSA, some wlil have only sleeping car passengers at one reservation time, and coach passengers at a time 15 minutes later or earlier.
In my 40+ years of traveling Amtrak and eating in the diner, I can only recall a very small number of times I've been seated with antisocial tablemates. However, I have encountered a few times when getting them to talk required some direct questions like 'where are you headed?' etc. Actually, even if my tablemates aren't that talkative with strangers, I am. As a result, I too-often monopolize the time and often end up last one finished eating. For me, my biggest fear is being seated with a toddler on my side of the table and a parent that doesn't 'keep them in line'. Fortunately, I've never had that bad luck.