The E line is the most varied of the T's trolley lines. It runs in the street, in a private reservation, in the subway, and on the elevated. The Heath St. Loop was built in 1945 to replace a cutback near Brigham's Circle. The old cutback was a set of crossovers and could not be used by the new PCC cars, which were single-ended. The line originally went all the way to Arborway, and many railfans still refer to it as the Arborway line. In 1985 service from Heath St. to Arborway was discontinued. There were many reasons for this, but the main one is that the T's upper management hates street-running (they'd never admit, though). Restoration of the line to Arborway has been in litigation on and off ever since.

From Northeastern to Copley Junction the E line uses the Huntington Avenue Subway, which was completed in 1941. Prior to that, cars continued along the surface to Boylston Street, where there was a portal. The location of the portal is still obvious if you know what to look for: a widening of the median. There's a similar widening in the subway below that (which provides room for a short siding that usually holds the rerailer car, 3417).