When we went to Washington, DC about 20 years ago, we stayed in what I believe was a KOA with huge wooded pull-through sites. I can't find it again in my searches - does this campground ring a bell w...
We stayed at the KOA in Millersville, MD to visit friends in Pasadena a couple of years ago. It was about equidistant to Baltimore and DC, but I have no idea if a train station is nearby, but they do have some wooded sites.