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A big issue on most Amtrak trains is they ride on tracks owned by the large freight railroads. The condition of the tracks varies from good to marginal, in terms of providing a smooth ride for passenger rail cars. For example, when I lived in Orange County, CA I occasionally rode the Pacific Surfliner train to or from San Diego. The condition of the tracks is pretty bad, lots of side to side rocking, loud metal to metal contact at every track connection. I can tolerate it for a couple of hours, but a whole day and night or longer, no way.
When I am in Germany, I ride trains a lot. The main lines have the tracks aligned very smooth with welded seams at the track connections. Side to side movement is almost nil, and there is no loud "clickty clack". This is to permit operation of high speed trains (240 kph / 150 mph) on the tracks that are shared with regional and local passenger trains. You can set a full beverage glass on your seat back table, and not worry about it spilling or falling over.
Due to high speed rail routes that shortened travel times, sleeper trains are obsolete within Germany. There are several that cross Europe. The best known is the Paris-Frankfurt-Berlin-Warsaw-Minsk-Moscow train operated by the Russian Federation railroad, a 39 hour trip. In Moscow, you could transfer to the Trans-Siberian express that runs to the Pacific Coast of Russia, or to Beijing. Either of those destinations would take 6-7 days. There are also overnight sleeper trains from Berlin or Munich to Budapest, and Munich to Croatia. I'm sure there are bunch of others that can be found on a google search.