Up the river from the concrete slab, they actually
used to harvest ice from Cow Island Pond in the
Charles River, near where the high school is now.

Check out this old map, top left corner for the
Highland Ice Company. Apparently, a spur of
the railroad that ran in parallel to Spring St.
cut towards CM, from in back of what would now
be Star Market:

communityheritagemaps.com/.../westroxbury30.html

Pretty cool maps.