The entrance to the Southwest Corridor is across Dartmouth St from the Back Bay Amtrak/T Station.  As the corridor passes a number of T stations on it's 4.7 mile trek you can go as far as you want and then catch the T back.

This panorama was captured with with the iPhone 5 mounted to a tripod with the Gymbl.  Composed from 36 seperate images, it was stitched together with Autopano.