From Dave Brigham:
I was walking down Mass. Ave. in Boston with my son, Owen recently when I noticed a small construction site right outside the Symphony subway station. We've ridden the subway a LOT over the years, but this was the first time I'd been to this stop. I realized right away that this must be a former entrance to the station; the current head house is about 30 feet away.
Owen and I went down into the station and he used his phone to look up the history of the station. He told me, thanks to Wikipedia, that there used to be a "sub passageway" connecting the inbound and outbound sides, but that it was sealed off in the 1960's. I looked at the Wikipedia entry as well, and learned that the MBTA did moderate renovations to Symphony in the 1990's.
Searching online, I found a random guy's photos posted on Facebook and Twitter of the same construction, and he indicated that there is some sort of utility replacement going on. He said the original Symphony station entrances were covered over when two large apartment buildings, Symphony Plazas east and west, were built, I'm guessing in the 1960's.
Here's a capture from Google Maps before the construction began, showing no sign of the old station entrances:
The current utility work, or whatever it is, is taking place right behind where the person is walking in this photo.
I can't find any old photos online of the original station head houses, but I'll keep looking.