via Flickr http://flic.kr/p/tfXFsX
You can see why, in later years and when the various lines of the old BRT were merged into the MBTA they adopted "The T" as a snappier name! BRT attempting to cover all bases in the title here, and this diagram excluded the traditional streetcar and ‘bus lines that existed. Some of what is seen here still survives in use, notably the subway now much expanded and with more traditional ‘underground’ lines added, with some changes such as the abandonment of the Tremont St subway entrance as the city’s streetcar system was laregly abandoned. Also gone are the ‘true’ Elevated lines such as the Atlantic Avenue line.