Once upon a time Boston's Orange Line ran on elevated tracks above Washington Street from Chinatown all the way to its southern terminus at Forest Hills.  In 1987, 16 years after several Boston neighborhoods organized to prevent I-95 from cutting through the city, the Orange Line moved to the sunken Southwest Corridor (which had been cleared for the highway) and the elevated tracks were taken down.

If, like me, you never exprienced the old elevated Orange Line, I came across some cool clips on Youtube that'll give you a pretty good idea of what it was like.  Honestly, it looks pretty fun.  Also, bear in mind that this is the same route that the MBTA claims is now equally served by the Silver Line BRT.

Orange Line inbound (Forest Hills to Chinatown):

Outbound (Chinatown to Forest Hills):

And going much further back...taken from a trolley in 1903, I believe this next clip begins at about the intersection of State St. and Washington St. and heads toward Downtown Crossing (let me know if you think I'm wrong here) before skipping ahead to Boylston St. and cutting through Copley Sq. (thanks to Loaded Gun via Universal Hub):