When originally proposed way back in the Williams administration, the Anacostia Streetcar was to use the old CSX ROW - abandoned after 9/11 - to run the streetcar on. That ROW would leave enough space for a parallel trail. But then CSX asked for an incredible amount of money and there was some legal concerns about whether or not they could even sell it. So the streetcar was moved to streets and the trail was dropped. Then, DDOT came out with 10 alternatives for the streetcar - one of which retained the CSX ROW.
Recently DDOT pared that list down to 2 alternatives, one of which would use the CSX ROW and include the parallel bike path. In addition to the path, that alternative (a Beatles-worthy number 9) seems to be better across the board - but for the current ownership by CSX and the conclusion that it is less accessible to the service area.
You can see the route and a cross-section here.
I'm not sure how much of a change this represents. Everyone I've ever talked to at DDOT about this has always said they would rather use the CSX ROW, but they're prepared to move forward without it. Perhaps identifying this option as one of two preferred options will create greater impetus. We'll see.
Even if the streetcar doesn't use the ROW, a trail could still be built along it later, but like the Purple Line one has to wonder when the political will would exist. If DDOT can't make the deal work with the streetcar and trail on the table, it seems unlikely they'd do more to get just the trail. On the otherhand, getting the Anacostia section of the old ROW turned into a trail, might spur (see what I did there?) DDOT to extend it all the way to C and 33rd Street SE.