As far as I know you can't legally hang in Germany ;-)
Not sure about Belgium.
In Holland (if, for whatever reason ending up there), only legally at campsites like "Paalkampeer plaatsen" (a central stick in the ground defines the campsite, if lucky with a water source nearby, usually no fire).
Most 'regulated' campsites in Europe are geared a lot more towards ground dwellers and caravan/camper and are, again, afaik, not easily reached by train.
But then again, I never really sorted out all the information.. Most times I just end up in the Scandinavian forests..... away from (cultural) sights....
And keep us posted about the trip...