Some of the older regional trains still have compartments, though they are being phased out. Newest trains are seats facing forward. But if you're lucky enough to get a compartment, and to get it for yourself/and group, this is a very comfortable way to travel. No way to predict train configuration, but I think the issue is worth getting first class travel. Compartments are in both first and second (for an older train that still has them), but it's more likely you can find a vacant compartment in first. Very, very comfortable way to travel. Beats driving, and is more informal than plane travel. It's also nice to take on board sandwiches bought at the train station beforehand, and just talk and enjoy lunch while traveling. Of course, some trains have restaurants on board, but many do not.

One very important point, though it probably seems too trivial to mention: Always, always take a bottle of water with you. Sometimes it's impossible to buy water on the train, and other times it is a mineral water that may not satisfy thirst. I hope this reminder won't strike people as too obvious or too silly. That bottle of water can seem important after a few hours of travel.