Thanks, all - with a special thanks to @tonfromleiden for the very useful link to Man in Seat 61. It's perfect.
And, yeah, I know it's a long train ride but we really do enjoy train rides (and not all that fond of flying). We rode from New York to Seattle a few summers ago on the Amtrak (without a sleeper) and enjoyed it immensely. We did include a two night stop at Glacier National Park in Montana. The train stops directly in front of both the west and the east entrances and there is lodging within a few blocks. (But be sure to reserve as early as possible.) During the summers, you can hop on the free shuttle buses to get around the park or hitchhike around the park - which we did quite a bit. Met some really nice folks that way and beat the crowds that can form at the shuttle stops.
But I digress...we also prefer trains so we can minimize our carbon footprint and as a way to see the countryside.
And we'll likely take the suggestions to stop over one night in Germany.
Thanks again!
(And I'm sure I'll be asking more questions as I continue to plan. This is a great resource!)
Judith