Prague to Amsterdam via rail - any good ideas?

How about Lueneburg, not too big a detour on the way between Berlin and Amsterdam.

To fit your description of an "old " town, Lüneburg can be done but you'll have transfer in Büchsen from Berlin or take the ICE Berlin to Hamburg, then transfer to Lüneburg, one of my absolute favourite towns in Germany. This also depends on which route you're taking to Amsterdam from Berlin (if you are tailoring this route)...through Hannover or via Frankfurt.

If your route is leaving from Berlin, then I suggest Minden an der Weser, hardly any tourists go there except Germans. I like Minden, have spent a few nights there. Take the ICE to Hannover Hbf, transfer to Minden, an "old" historic and garrison town where the "Mittellandkanal" connecting the Elbe and Weser, a town with half timbered buildings, (Fachwerkhäuser), a real scenic Altstadt, plus the Prussian Westfalen Museum and battlefield site of Minden. Minden is not diversion en route to Amsterdam, Lüneburg is. The IC train stopping in Minden has its terminus in Amsterdam Centraal. That's a direct shot for you.

Thanks for the input....I guess I need to find a more detailed map as neither town shows up on my Rick Steves map. I'll definitely pursue!

Not surprising that these two towns don't show up on the RS map given his focus on Germany. Neither has a train station classified as a Hbf.

@ sonidavison...Another cityie, larger than Minden or Lüneburg, and one with a Hauptbahnhof that is not too much of a diversion "for cross Germany travel" is Münster/Westfalen. Certainly not a city swamped with US and international tourists. I suggested Minden since it fits in your route logistically but if you want to flex the route and logistics, then Lüneburg is much more interesting historically and culturally. Lüneburg is one of my few favourite towns in Germany, Minden is not.