The following cities have a convenient (sometimes night train) connection, only ONE change needed at the end of the trip: Amsterdam, Berlin, Bologna, Budapest, Dijon, Dusseldorf, Frankfurt, Genova, Graz, Hamburg, Hannover, Köln, Leipzig, Ljubjana, Milano, Munich, Paris, Salzburg, Stuttgart, Utrecht, Vienna, Zagreb (all as per spring 2024).
And here is a small selection of cities from where you would have to change trains only twice: Barcelona, Belgrade, Gdansk, Kopenhagen, Lyon, Marseille, Prague, Rome and Warsaw.