This is 100% what I saw at PVD. Employees (most in uniform or with Amtrak branded gear at least) occupying seats like paying customers, but conductors were standing at doors like it was a normal in-service train. The signs above the doors read "not in service," however, so it was pretty clear what was happening. I think it was holding at the station for a bit longer than normal because there pretty extensive delays Tuesday afternoon (downed catenary wire just north of PVD station and heat delays further south) so I walked the length of the train to peek in.This might've been what you saw:
"Dress rehearsal" runs open to Amtrak employees simulating paying customers.
Looked like it would be fun to be an Amtrak employee on these dry runs. I'm not sure what rules there are (if any), but they seemed to be enjoying it. The cafe car wasn't operational (lights were on, but all of the surfaces were wrapped in plastic), and there were boxes of KFC all over the place. Looks like they had their pick of sodas and beer too. Not a bad way to break in the new trains.