We stayed at the Double Tree in Silver Spring last time we were there. Nice place, about a 5 minute walk to the Metra (I think it's the Red Line out there), some decent places to eat within walking distance, pretty clean, safe, decent area.
Also have stayed out in Old Town, Alexandria just off the Metra before too. That was nice also. We just looked for good prices on hotels in decent areas and near the rail. We liked either place.