Hey obn,
I just ran that trail last weekend. Here is the map that I created from it:
http://www.mapmyrun.com/run/united-states/dc/washington/159124862760125232
I know that we started at the southend trailhead, off of the Capital Crescent Trail and then my instructions were "run up the trail until you hit Massachusetts Ave, which is the 4th road you would cross". Meaning that you can enter it from Capital Crescent / Canal Road, then 4 other places, with the last one at Massachusetts Ave. I don't know if the trail goes past there though, cause I then turned onto Mass (as seen in the map).
Mike