Boston has been scrubbed clean over the years of its miscellaneous unused transit infrastructure. In particular, the elevated railroads nearly all are gone. Most recently, the sun shined on Causeway Street. In the summer I stumbled onto one of the pieces of unused transit infrastructure that hasn’t been removed.
This tunnel entrance is located just southeast of the Fort Point Channel, facing the Red Line railyards. It could be a lot of things. I have a hunch that it is a trolley access for a disused upper level in Broadway Station. Apparently several of the Red Line stations were built with trolley mezzenines that since have been abandoned. If that was the tunnel’s use, then it wouldn’t ever have had a direct link into the Red Line tracks.