[The small Masons were used on the BRB&L too before that line was electrified.]
The Boston, Revere Beach & Lynn was a three-foot gauge line. As such, their coaches were close to normal size. Note also that after electrification, the BRB&L acquired a few Boston streetcars which were re-gauged down from standard gauge to provide mid-day service. Finally, after abandonment, the right of way from what is now Airport station to what is now Wonderland Station was rebuilt for the Blue Line subway.
It is my understanding (I could be wrong) that the BRB&L coaches were acquired by the East Broad Top RR (also 3 foot gauge). Note to somebody else, those were wood coaches, not heavy steel construction.