Check the Brunswick line train schedule using the link provided in "Resources." Trains depart at various times through the day.
Arrive at Union Station in Washington D.C. roughly 30 minutes before your train is scheduled to depart. This will give you time to get tickets and board the train.
Purchase a one-way ticket on the westbound Brunswick train. As of 2010, the fare from Washington to Martinsburg, West Virginia, was $10.
Board the Brunswick train and stay on it until you hit West Virginia.