Well, Mansfield does have a mini-high. But since again the Acela I was riding went back to NY, it doesn't really matter what level the platform is at. Regionals can serve either.
As for why, all I know is that dispatch said to stop & pick up passengers that were stranded by the problem. Don't know if they were Amtrak pax that got dumped there or MBTA pax that were stranded since there were no T trainsets west of RTE. I've passed 1 since leaving PVD going west, but it would probably be at least another 1/2 hour or more before that trainset heads back to Boston. Longer if he's got to go west of PVD.