They're building this to ease Worcester Line congestion that's happening right now, because Purple Line trains right now aren't clearing off the stubby side platform in time for the next slot. In part because the existing 350-footer full-high can only platform doors on 4 cars at once, and Worcester already runs 7-packs at rush making that platform an all-around horrible dwell. Building the new island at NE Regional-spec 1000-footer (12 cars) instead of T-spec 800-footer (9 cars) is 95% the serendipitous result of needing to align the north stairs parallel to the kiss-and-ride lot on the other side of 290. That's just where it happens to be...at the 1000 ft. mark instead of 800 ft. They'd have to lop the employee lot in half to move the sidewalks around in another egress configuration to take 200 ft. off the platform length, and that was more construction touches than truly necessary so they aren't bothering. Because the stubby side platform pinches at the 290 overpass to less than regulated ADA length, they're leaving it stubby. Only the first 4 cars on Track 1 will be able to open all-doors. The island is thus going to become the primary overall platform.So, we need TWO Amtrak spec platforms for the Lake Shore Limited?
The remaining 5% consideration is future-proofing is for the Inland Route. Should a Springfield-terminating Regional get routed east (distinct from a NHV-SPG-BOS Shuttle making up the bulk of the schedules), the maxi-Regional length platform is already there at Springfield and Worcester for convenience. Nothing routed specifically on the Inland--Shuttles, LSL, Boston-Montreal, future Western MA & Albany--is ever going to run that many cars. Only a thru-routed DC-SPG Regional at prime time slot...but that could be a bit player in the overall schedule churn.