#1  Overnight trains cost more! – Most European night trains are included in the Eurail passes, but you still have to reserve and pay for the type of sleeping accommodation you choose.
#2  You have CHOICES about what the type of room you want! – If you simply want to spend the night in a regular seat, you simply reserve your seat and away you go. However, if you choose to stay in a private room – with one to six beds. You can actually rent a single bed in a six-bed cabin, and this will cost you much less than if you want the luxury of having the room all to yourself, or with just enough beds for your family. I have stayed in a six-bed cabin, and found it quite tight for space, but doable. Otherwise, I get a room with just enough beds for my family – which I prefer, and it is much more spacious. Some rooms actually have their own toilets, which saves you having to leave the room in order to do your business.   
#3  Overnight trains can be worth the extra cost! – It’s simple – the more beds in the cabin, the less you’ll pay. You can sometimes get luxury rooms with your own toilet, which will cost more, or you can opt for a room with a sink and vanity area only – which is very convenient for brushing teeth, doing makeup, etc. There are also rooms that simply have beds, and those are more primitive, and cost less than the personal-sized spaces.
#4  Overnight trains are safe! – I remember hearing terrible stories about overnight trains, but after years of using them and not having a single safety issue, I believe it is like anything else – just use common sense and be aware of your surroundings. The doors to your cabin lock very securely, and it’s impossible to open them from the outside. Never open your door for anyone who is not a Eurail employ, police, or customs officer. If you have to go to the toilet during the night, use the buddy-system and don’t go alone. Each overnight train I’ve been on has an attendant who is awake all night and is there to make sure everything goes smoothly. He will introduce himself to you when you board, and if you need anything during the night or have any safety issues, you can find him easily.
#5  Overnight trains are LOUD! – One of the biggest surprises I had when I took my first overnight train was how loud they can be! Throughout the night, the train stops – sometimes for long periods, as well as adding or dropping carriages. I suggest earplugs, and I’ve downloaded a ‘white noise’ app that I use to drown out the various noises throughout the night.
#6  Overnight trains are a great way to make new friends – Not all trains offer a dining car, and your only choice is to head straight to your room, where you’ll spend the remainder of your time on the train. In this situation, you won’t be meeting many folks. However, some trains actually have a dining car, even if it’s only open for a few hours after leaving for your destination. On our way to the Arctic Circle last year, this was the case, and I met so many amazing new ‘friends’ that I stay in touch with. I simply asked a couple if they’d been to Norway before, and that spurred two hours of conversation, with several other couples and fellow-travelers joining in. We laughed, shared travel stories, and eventually exchanged contact details. We are still in touch today. So, make sure to talk to people when you get the opportunity.
#7  Overnight trains can be late – Although most overnight trains are on-time, it is not unheard that overnight trains run anywhere from a few minutes to a few hours late. Therefore, if you have a connecting train, keep this in mind.
#8  Overnight trains have terrible pillows! – This is a MAJOR issue to me, and I’ve never heard anyone compliment overnight train pillows. They are usually flimsy and lifeless, and you have to fold them in two (or three) to get enough volume to support your head properly! Any time I’ve ever been forced to use them, I wake up with a crick in my neck! If this is not YOUR experience – CONGRATS! I’m jealous! My solution is to buy a cheap pillow to carry around with me, and several pillowcases to rotate between washes. It’s well worth the price!!!