Hello Bellingham! We are from Anacortes and moved here a couple years ago :). I would stick with Dulles so you can fly direct. An uber from there to union station will be expensive (~$4 metro to $60 Uber)The silver line runs from Dulles. Take the silver to largo town center. Get off at metro center and take the red line to Glenmont. Then get off at the union station to catch the ride to Martinsburg. If you want to stay at a hotel near Dulles, there are a few right by Tyson’s stop (between IAD and DC) that also have lots of shopping/restaurants.

There are lots of benches for sitting and the metro usually runs every 15 minutes. So give yourself lots of time to catch your Marc, and take your time.

My mom visits us and studies a picture of the metro dc transit system to feel more confident :)