An extension to Chelsea from Maverick planned in 1924; wow, I never knew that. If not for the need for the Blue Line to Logan Airport, it would have made sense.
As for Maverick Station itself, I remember as a kid the old trolley car ramp and portal still open to the outside and used as a pedestrian entrance to the station, and also the loop at the north end of the station still being used by Blue Line cars. Why did they close of the loop? Seems like it served a useful purpose, although I suppose the Orient Heights yard probably replaced the need for it.
Well, don't forget the Boston Revere Beach & Lynn was still doing torrid passenger business during the Roaring 20's...back when Revere Beach was at its all-time economic & tourism apex. That was 'peak' all-time RR decade right there, and also 2 decades before the end of steam when all RR's were still technologically unidirectional tracks and unidirectional rolling stock, before some of those 4+ track mainlines could re-signal themselves to cram 4-5 tracks worth of traffic comfortably on 2-3 tracks and entertain selling off their slack space to side-running rapid transit expansion. Thus, for Boston Transit Commission it was still the base assumption that any/all expansion plans outside of city limits would forever have to be chasing their own homegrown ROW's because nothing owned by the RR's would ever have capacity to give. They were still squarely in "Els everywhere" mentality, and only beginning the fight over extending the Orange Line to Malden on stilts over Main St. because the stone's-throw adjacent Saugus Branch AND Western Route were off-limits.
Obviously within little more than a decade the whole world flipped on its head, and after a 1930's of total bloodletting the still-independent BRB&L was out-of-business by 1940 with the state buying the entire ROW. The Maverick tunnel stub never would've been used at any point further for reaching Chelsea, because some combination of under-utilized RR ROW's would have accomplished exactly the same task. For example: BRB&L to Logan & Wood Island on the current Blue alignment then vulturing some surplus side tracks from the gigantic East Boston cluster of B&M + B&A port freight tracks that used to take up the entirety of Route 1A's width footprint would've accomplished the job at getting them on Chelsea's doorstep. Then take it from there on whatever tunneling or El tricks would've gotten them downtown. The 30's and early-40's were a Crazy Transit Pitch-a-thon of crayon-drawing schemes like that utilizing any/all ROW's (both the availables and not-yet-availables). And then the Alternatives were sifted through and busted down to the famous 1945 expansion map, which rolled up its Blue Line vision into straight-on linear expansion over the whole BRB&L mainline to Lynn.
That Chelsea-leaning tunnel stub thus never would've seen any use unless an extension had been greenlit shovel-ready fast!-fast!-fast!
within 5 years tops. Because post-Crash the planning pivot was immediate to seeking synergies with all these underutilized RR ROW's. Keep in mind as well, HRT conversion to Maverick was a top priority almost since the East Boston Tunnel opened in 1901, so that stub-out design was probably sitting on a shelf close to 2 decades before Maverick was actually rebuilt and they thought they would
be looking at multi-decade staging of a tunnel + El buildout. The big slowdown came from opposition in Cambridge to doing a Red Line El run-thru into the Eastie tunnel @ Bowdoin, and the multiple plan revisions that eventually let to a Charles Circle reverse-branch junction for either Maverick or downtown that would've been interconnected by other Els to other places later on. It was when Red was shovel-ready in 1912 and Blue was still stymied in conversion politics that all those considerations fell through and they just cleaved the networks off to run their own different-size HRT rolling stock. Had things not ground to a halt in Cambridge and ended up getting the HRT trains to Maverick by no later than the 1910's planning off that Chelsea stub probably would've been a hot topic. As it turned out, the opening delayed to 1924...the giant Crash happened only 5 years later...and the RR's were immediately wiped out starting the huge planning shift from Els to slack RR ROW's.