If that's a concern, that train left the station a long time ago, so to speak. Amtrak conductors routinely "mass lift" all the tickets without scanning them individually, and there's rarely an accurate count of passengers on many trains.
Yes, I do believe that happens. But I suspect their insurance carrier will only pay out for ticketed passengers and then only after some proof they were actually aboard. It's tough to collect death benefits for yourself (unless dealing SSA).
Again, heaven forbid the need.