Yes, this. Take the Marc train from Union station to Martinsburg WV. M-f first train leaves at 3:30pm last train leaves at 6:20pm. If you miss that you have to transfer to a bus, see timetables. If you want to make the trip a little easier consider flying to DCA. You will probably have to change planes at a hub like Chicago or Denver but DCA is so much closer and easier to travel through. It will be worth the layover and the extra expense so you don't need to take a taxi or an excessively long metro ride. Also DCA early afternoons is not an expensive taxi to Union station if traffic is light. Traffic gets bad at 3:30 from DCA coming into downtown. After 4:15 definitely take metro from DCA to Union station.

If you get a hotel and want to check out some of what DC has to offer just Priceline it and anything but the red roof and the days inn will be nice. DuPont / Connecticut avenue / foggy bottom are nice neighborhoods to name a few.