Jeez...
They pre-board at South Station because there are often several Amtrak trains laying over at a time, and they don't post the track number until 15 minutes or so before departure. If the weather isn't horrible (and sometimes even if it is) people start to que up on the platform a half hour before the track number is announced. The platforms are also directly accessible from the street, so if you're getting dropped off there is a good chance you will never have seen the board in the first place.
Typically there will be two or three Regionals, an Acela or two, and even the LSL sitting there, plus MBTA trains pulling in and out. Lots of opportunity for people to board the wrong train. Having someone check tickets is a lot easier than dealing with an Acela passenger on a Regional or vice versa, or worse someone getting on a train and taking a ride through the wash racks and into Widett Circle.
Back Bay and other through stations are different. The track is announced, it pulls up, people get on, and it pulls out. I doubt it has much of anything to do with security, although I guess while the train is sitting idle there is the chance for someone to sneak on and do something so it may be a small deterrent. I can't speak for the crew, but I assume its also easier for them to get their jobs done readying the train for departure without passengers boarding early, asking questions, and generally getting in the way.
Also,
boblothrope wrote:
Care to provide a cite for an actual incident where someone didn't know the difference between an MBTA and Amtrak train they were looking for?
This happens
all the time. I've been taking the train between BOS and NYP every three months or so going on a decade now. At last four times I've personally overheard conversation between the conductor and a confused passenger with an Metro North/MBTA/NJT ticket who has to get dumped off halfway down the line. If I've seen it happen riding relatively infrequently, I can't imagine how often it actually does happen.