This is what the
South Station Expansion DEIR says about mitigations to the Tower 1 chokepoint.
View attachment 49810
Basically, it's the very slow speeds that lead to a lot of crossing conflicts. If the trains cleared the switches at 25-30 MPH instead of 5-10 MPH, there'd be a lot more flexibility as the switches could cycle faster for differing directions. It's not an unlimited bump, but it stays ahead of Regional Rail traffic growth. And Cove + Broad have more potential for speedups than Tower 1, so the planned speedup interlocking mods to those two take a considerable load off Tower 1.
Beacon Park layover just doesn't provide enough benefit being cut down from a dozen trainsets to 8 (with relentless pressure from Harvard to keep cutting), and with :15 minute service + expresses + a much-expanded expanded Amtrak schedule squeezing through Back Bay and Lansdowne the slots are going to be very constrained for deadhead moves during shift changes. You can't be burning a few TPH through there on strictly deadheads. It pretty much wouldn't work during hours when Amtrak has a slot, meaning you'd be diverting to Widett at certain conflicting times regardless. Had they done the original Widett-less layover plan that spread the expansion capacity around BP and Readville, those conflict slots would have to divert to Readville or the nooks-and-crannies around Southampton that they already use...meaning Tower 1 gets stressed regardless. It was very traffic-limited, and indicative of how much they were grasping at straws for space anywhere they could find it before the centralized Widett parcel fell into their laps. It's definitely a better ops situation now with the current layover plan.