The plaque and stone memorial sit beside the old MBTA headquarters at the Arborway bus yards, along the Arborway and the Casey overpass.
Carmen's Memorial (BPL)
A tablet honoring the 98 members of the Jamaica Plain carbarn who served in the World War. In front of hundreds of Jamaica Plain residents, a new flag was raised, while the 161st Regiment Band played "The Star Spangled Banner."
A service was held in St Thomas church. After prayers and speeches by various dignitaries, colorbearers led the gathering to the carbarn, where the bronze tablet, set in a large boulder, was unveiled. Of the 98 men who served in the war, five gave their lives.
So where is this tablet now? I seem to remember a boulder with a tablet in it at South street and the Arborway. Did it go to the American Legion post? I just don't remember a World War I memorial. I'll have to ask around.
Addendum: Great news! The Boston Public Library is making their photo collection available on Flickr. The photo above is one of the first group online, with more on the way.
Addendum 2: I found the memorial at the old MBTA headquarters while driving through towards Forest Hills - pure luck!
Source: Boston Daily Globe January 26, 1920