by edbear
I was working for Amtrak during that era and the MBTA had the final say on what operated where. Some of the high maintenance equipment was moved to the North Station routes as there was an enclosed site where they could be maintained. Also, the passenger volumes out of South Station were astronomically higher than out of North Station, so longer trains, or higher capacity - bilevel equipment- were necessary. Some North Station lines could not use bilevels until some clearances had been improved.