The better-known part of Boston's light rail network is the Green Line, which consists of four different branches that go under downtown in part through America's oldest subway.  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. 
   
West of center city, the subway continues under Boylston Street before the first branch splits off for Heath.  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 other two branches go to Boston College and to Cleveland Circle. 

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

In addition to the Green Line, there is also a short light rail route known as the Mattapan High Speed Line.  It serves as an extension to the Ashmont Branch of the Red Line subway, and has no connections to the rest of the trolley system.