Mystery site. (Copyright 2007, all rights reserved)
So where were these pics taken? Hint: it's at the edge of Jamaica Plain, and it's public property.
We have a winner! Pagel park is between the railroad tracks and Hyde Park avenue opposite Wyvern street. That puts it right at the edge of Roslindale. The tunnel came out at the south edge of the Archdale housing projects where Brookway terrace ends.
Stony brook once crossed Hyde Park avenue right at the north edge of Pagel park and went under the railroad tracks. A City of Boston street directory of 1955 lists a Brookway Footpath between Brookway Terrace on the west side of the tracks and Hyde Park avenue on the east. This must have been it. The Archdale side is totally blocked off, but the stone wall on the Pagel side is still in view if you walk up close.
Congrats to Massmarrier - I buy you a virtual beer!