The main part of Boston's light rail network is the Green Line, which consists of four different branches.  All go under downtown in the Tremont St. Subway, America's oldest underground transit route.  The heavy traffic and the multiple destinations result in headways often only seconds apart during peak times, and in very crowded conditions at the old stations. 

Eastbound these routes end either underground at Government Center or North Station, or at the venerable outdoor terminal at Lechmere, across the Charles River in Cambridge. 
   
West of center city, the subway continues under Boylston Street before the first branch splits off for Heath St.  The three other lines emerge to the surface after Kenmore.  The longest route extends all the way out to Riverside, in large part along former railroad right of way.  Its opening in 1959 was an early milestone in the rebirth of U.S. light rail.  The two other branches go to Boston College and to Cleveland Circle. 

The view on this page, through a front window, shows another car just ahead and an outbound train approaching.  The subway portal and Kenmore station are not far ahead.  The Prudential Building is in the background.

In addition to the Green Line, there is also a short trolley route known as the Mattapan-Ashmont Line.  It serves as an extension to the Ashmont Branch of the Red Line subway, and has no connections to the rest of the light rail system.  And it still uses those glorious old PCC streetcars.