Boston has always had state-of-the-art control centers

It is a rotary converter/motor-generator set, but not a particularly huge unit. I'm guessing it more likely powered a trolley line (e.g.: a part of the Green Line) rather than a heavy-rail rapid-transit system. Searching Google shows a number of occasions where such equipment was purchased for the Boston Street Railway from the General Electric Company of Schenectady, New York.

Judging from the buss bars, I think there's a second unit positioned behind this one. It's curious that this machine is installed in a room with such a low ceiling. Power rooms usually had much higher ceilings so that components could be hoisted in and out of position for maintenance and repairs. This looks like it's the basement of a larger building, rather than a stand-alone powerhouse.

This video from New York City has pictures of similar converters; both larger and smaller; accompanied by an audio recording of one starting up:

      ^{Note at time 0:37, a quite similar General Electric Company converter.}