The Cleveland Circle branch, also known as the Beacon Street line, is one of the oldest trolley lines in the US, dating to 1889.  Running down the median of Beacon Street, it is one of the more scenic lines in the Boston area, with the buildings along the street having lots of interesting architecture.  The line retains many features of early trolley lines, such as closely spaced stops (many are less than a block apart, and there are places where you can look down the tracks and see two or three stations) and frequent road crossings.