Boston's MBTA subway Green Line is one line between Lechmere Square near the Museum of Science in East Cambridge through Park Street Station (beneath Boston Common) and central Boston, to Copley Square, but west of Copley it splits into four lines.

Green Line trains can take you from Boston Common to Copley Square, the Hynes Convention Center (and Boston's HI-'Youth' Hostel), Kenmore Square, Symphony Hall, the Museum of Fine Arts, and the Isabella Stewart Gardner Museum, but you must be careful to board the right one of the four trains (B, C, D or E) to reach your destination.

Outbound (that is, going west and south away from the center of Boston) Green Line trains emerge from underground and serve as trolleys after they leave the central part of the city.

Here's a map of the Green Line routes.