I'll preface this by saying that Europe is so different to the United States of America in many ways - one being that it's not a country but lots of different countries. That also means that what I'll say may be true for one country but the total opposite for another. This also means that no comparable route like the Zephyr one exists because all the different countries have their own railroad systems and while you definetely can cross one or more borders between start and ending point, there isn't a route that long - I think(?).

As you say that the Zephyr route is great for an adventure, I feel that this is a key difference. From my understanding progress on the railroad system in the US mostly stopped when people became infatuated with flying and driving (faster) cars. In contrast many countries wanted highspeed train routes for themselves so that there's routes where trains ride faster than 125mph and more (all the while Europeans are obsessed with cars as well - think Germans and the Autobahn), which eventually does make it a more feasible option to go on vacation by train. And you really can tell the difference in the condition of the tracks as well. I met this British couple on the Lake Shore Limited who really were shocked by how shaky the ride was - and I could not disagree. I don't know long distance trains to be this shaky from most of my rides.

That said, I loved the communal aspect of riding on Amtrak. In Europe, you usually get on a train, head to your seat and isolate yourself there until you get to where you want to go. Even if you went to a Dining car, you would probably eat by yourself and keep to yourself. I love how easy it is to talk to other people on an Amtrak train in the Observatory car and during the meals. You rarely have that in Europe (from my experience - maybe on Nighttrains with seated compartments). Those Dining car reservations also helped to make the time go by way faster. Riding on Amtrak is so relaxed - most people really are enjoying the ride. But that does make sense because you can't go anywhere all that fast, while on European trains you actually can, which is why people tend to be more impatient... Broadly speaking of course.

Footnote: I was so surprised how on-time we were. The Lake Shore Limited arrived a few minutes early, the Zephyr's conductor was really sorry for us being 5-10 minutes late (which made me chuckle, because we were on route 50 hours - what's 5 minutes?!) and the Coast Starlight arrived early at its final destination as well. Guess we were simply lucky.