Economy is a big reason to take a Europe trip this way, but maybe more important is the fact that your travelling is much, much more efficient. You can see a lot more in a given period of time because the hours spent getting from one place to another are nighttime hours and in the daytime you are sight-seeing, enjoying the parks, etc. rather than looking for a room, packing and unpacking, and such. Those nights you do take a room give you plenty of time to do your washing, relax, and get ready for the next move. It makes all the difference in the world in how you feel. If you do this 45 minutes before you get into a town in the morning, the washrooms are normally empty.
First class has six seats and a sliding door.
In many trains, the bottom seat cushion pulls out and they all meet in the center of the compartment, making a flat surface about six and a half feet square, very comfortable if there are only three people. If not, by taking seats facing each other, you can sleep feet to head. Only very rarely have we had to sleep sitting up. September and October are ideal. In the fall, do the north countries first and work south as the cool weather comes on.