It's not very time or cost efficient to get to the PHL airport from the DC area any way other than by car (your own or a rental). I recall there are some buses from DC to Philly that I suppose you could take, and then get on that R1 train line. Amtrak also works, but as noted above, Amtrak is overpriced and not terribly convenient once you have to switch to the Septa train.