From Dave Brigham:
Located across from the above-ground Science Park trolley stop on the MBTA's Green line, this little building may lead to a fantasy world where the Kingston Trio's "Charlie on the MTA" plays all day and night; Alice, the Cheshire Cat and the Mad Hatter still use tokens; and the trains actually run on time.
I checked it out one day while on a subway excursion with my son, Owen. I'd spotted this odd brick building numerous times, and just had to take a closer look. Situated on the very busy corner of Storrow Drive and Martha Road, directly in front of Whittier Place Condominiums and the Clubs at Charles River Park, this squat edifice contains mysteries, of that I'm sure.
My best guess is that this is an entrance to the subway tunnel that goes underground just a short distance away, heading toward North Station. A Wikipedia article about the former Tremont Street Subway indicates that "[t]he northern portal at Canal Street was replaced in 2004 when the subway was extended beneath North Station to a new portal next to Martha Road."
Now, if you'll excuse me, I'm off to a tea party.