I am also planning a trip from Pittsburgh to Washington DC this coming year, probably in April 2013.
I found someone posted a GPS track in gpx format at this link:
http://ridewithgps.com/routes/1085023
The track had over 10,000 points (or bread crumbs) and my vintage GPS only takes tracks with a max of 750 points. I used the filter in Mapsource to reduce the points to a quarter of what it initially had and then I split that track into 4 separate tracks.
The person that posted this appears to have left the trail at several points and switched to roads, I have no clue why. I learned this by zooming in on the map at that site and comparing that to the bicycle trails marked by Google. Perhaps there were trail closures when he or she rode the trail? Much of the deviation occurred in and near Pittsburgh.
Have a good trip. If you learn of a "best" route from the Pittsburgh Amtrak station to the trail, please post that.