I'm ignorant on the subject, as I wasn't really paying attention to this when they were in the planning, but why did they decide to construct the Union Square branch with this? GLX-Porter makes sense, and I understand all the reasons one would want Green to Porter and beyond, but at completion of this GLX, it seems the catchment area for the two branches will strongly overlap. Those directly in Union Square benefit a great deal, but how, on a cash-strapped project, did they justify adding this single-stop branch on?
In the original scoping studies >15 years ago it was proposed as a uni-extension to Tufts
inclusive with Union as an intermediate stop, using some Fitchburg ROW, some cut-and-cover under (Prospect St.?), and then deep-bore through the base of Prospect Hill to get back on-alignment with the Lowell Line @ the McGrath/Medford St. intersection. That scoping was also mode-agnostic, as one of the Alts. included a Blue Line scheme (obviously eliminated in first cut) where bang-a-right from Charles MGH and trenching under the Charles locks would've set the trajectory. You'd be able to find mention of those early looks way, way at the bottom of the Project Archives on the GLX website. If not in full, then at least in a summary of most-recent previous studies inside the 2005 Major Investment Study archived there.
The inclusion of Union struck a difference from the 1945 straight-to-Tufts scheme and various look-see's after. Tunneling through Prospect Hill was always seen as very iffy, but they looked at it because modern Union Sq. projected a can't-miss on ridership worth giving it a try. The tunnel costs did upon analysis prove too steep, but Union's projected ridership also post-analysis really, really proved too big to just skip. So they opted for the compromise forked-branch scheme, which was one of the pu-pu platter of original study Alts. Given what they learned about Union ridership in the process, the course was set from there: it simply had to be included one way or another, and forked-branch was simply the best possible bang-for-buck for making it so.
Had things been studied earlier to rule out the uni-extension scheme earlier, you might've seen the forked-branch Alt. presented from Day 1 with Porter as a terminus instead of Union. But Union was within the original project limits, and they had to pivot quickly after the first-Alts. assessment thumbs-downed all the Prospect Hill tunnel schemes, so the MIS--working within the parameters of project limits--drew the forked-branch Preferred Alt. this way. STEP immediately crowned Porter as an eventual advocacy goal just like the Mystic Valley Parkway addendum, but given the way the project limits (funding, City of Cambridge, crossing of add'l Legislative district boundaries, etc.) would've had to expand more with Porter inclusion vs. MVP inclusion it wasn't mountable as a direct project Phase tack-on and thus is still an active advocacy rather than an explicit finish-up commitment like the MVP extension.
Agreed...if you were just joining today and didn't have a Project History summary handy the Union terminus would be a little bit of a head-scratcher. But it makes complete sense given the trajectory of events from first serious study to now, particularly what demographics they did and did not (i.e. greater Porter-universe) include in the Project Scope which informed what their most practical pivots were from 1st- to 2nd-round Alts. scoping.