Actually at South Station they regularly put a velvet rope across the platform until boarding begins, and once it does they check everyone's tickets before you can walk down the platform to board. This nonsense is done by Amtrak staff not Amtrak police, although sometimes an officer and canine is standing by, and can be avoided by simply having a red cap take you out to the train, which is generally available for red cap boarding at least 15 minutes before the silly red velvet rope is dropped.

Having said that, I have never had an issue using an RTE-BOS-BBY ticket for boarding at one of the other stations. I do sometimes find the first class attendant in a panic when I am returning from NY and getting off at 128 and my ticket said BOS and they think I am confused.