Once the principal powerhouse of the MBTA, this monumental structure has been converted to low-to-moderate-income housing. On a prime real estate site, this transformation is the result of collaboration between the City of Boston and the San Marco Housing Corporation. Units are grouped about a central skylit atrium, supported by a grid of steel beams. A colossal triad of arches serves as the main fenestration of the Commercial Street red brick facade, set between a diaper-patterned entablature and a drafted masonry base, with the whole echoing the neighboring warehouses.