At least from my experience, in Washington for regional, they will announce that handicapped and BC passengers can board first. So, if you are BC, you can comfortably sit while waiting, and then at the announcement, waltz right on by the long line of tired coach passengers.
The inconsistencies of Amtrak. I was boarding Regional 195 in DC this past Monday and there was no priority boarding for senior citizens, families or BC. It may have been due to the fact that 195 and the Silver Star were boarding simultaneously at adjacent gates so it was a madhouse.
Also, I came in on #30 and was not able to board from the Acela lounge. The attendant said to go to the gate at 2:15 for a 2:50 departure. At least I got to wait in the lounge until departure, #30 was 25 minutes early.