The garage shutdowns required turning back trains at North Station from the north, which the turnback is not designed to do. The early garage shutdowns had trains make an adverse move through the turnback crossover without signal protection, requiring that the interlocking be put on hand-throw and trains be flagged through double-red signals by an official. Because of the adverse move, only one train at a time was allowed between Science Park and North Station, resulting in such lousy headways. Later garage shutdowns got safety clearance to use a crossover between North Station and Haymarket, which vastly improved the situation.
Normal use of the turnback to turn trains from the south/west, as is happening with the upcoming diversion, does not pose such problems. The North Station turn-back has handled two branches full-time at several points in its history, including from its opening in July 2004 until the new incline at Science Park was completed and opened in late 2005, from April to November 2011 for accessibility renovations at Science Park, and from the closure of old Lechmere in May 2020 to the start of simulated revenue service on the Union Square branch in late 2021.