Very rarely do I praise TRE for anything, but I will say that Trenton does still have the same helpful system that Heading North (post #12) mentioned seeing a few years ago.
I am often commuting through there when the Meteor comes in, and they always announce the boarding locations over the loudspeaker, way before the train arrives, and you can hear it both in the station and on the platform. Plus they usually have an Amtrak agent on the platform making sure people are at the right location. They have a list with them, and make sure everyone on their list is accounted for.
Even the "regular" trains get announcements. They announce where the Quiet Car and the Business Car are on the regionals, for example.
The Palmetto is the most confusing--if you're going south of DC, you board at one location, DC or farther north, another location, and the Business Car and Quiet Car, other locations, all of which are announced--they practically run through the whole alphabet of boarding locations. I think if you are going to Savannah and want the Quiet Car, you are out of luck!