You'll want to consider your options because there's more than one way to travel by train in Europe.
I get a lot of calls from clients asking about rail passes. The general rule is that if your itinerary includes more than 2 legs of travel, a pass could well be the best option for you. Under two legs and a pass will cost more than the 2 tickets.
If a pass is the best option for you then you need to calculate how many days of train travel you'll need and how many countries you'll be covering. A 10 day trip won't require a 10 day rail pass unless you'll be traveling between cities every day. Just figure how many days you'll be going from point A to point B and that will determine how many days of travel you need.
With a rail pass you can make as many stops as you like for the 24 hour period on each day of pass travel. So if you are traveling between Rome and Florence you can stop in Pisa and/or Sienna on your way. With a point to point ticket, you can't do that. Passes can also carry discounts on ferries and local tours so pay attention to everything that's included with the pass. Note, rail passes can not be purchased in Europe so you'll need to make that decision before you go.
Seat assignments are a good idea. I've jumped on a train, gotten comfortable in my seat only to be bumped out of that seat at the next stop when someone with a seat assignment for my seat gets on. For long hauls, I recommend them. You can get settled in and know you won't have to move for the duration of the trip. On some routes, seat assignments are mandatory and will be included in the price of the ticket. If you're traveling on a pass, the cost of the seat assignment will be extra. You need to remember that if you're traveling on a pass it doesn't guarantee you a seat if the car is full so a seat assignment will ensure that you have a seat on that train.
Consider first or comfort class. These cars will offer more leg room, seat comfort and luggage space. On short routes it might not be important, on long trips I think it's worth the extra cost. Also, the different categories can offer varying amounts of flexibility should your plans change. There can be big differences in coach/economy class from one rail company to another. Once, I sat next to chickens in a cage in economy class and another time economy looked a lot like first class. Consider your expectations before you decide on the class.
Overnight sleeper cars sound like a good idea don't they? Save the price of a hotel room and arrive at your next destination, rested and ready for a new adventure! WHOA! I have never been able to have a good nights sleep in a sleeper car. If you can sleep anywhere it might work for you but unless you have the option of a private sleeper car you'll be sharing it with strangers. They can be noisy, hot/cold or just uncomfortable. Just consider what it takes for you to have a good nights sleep at home and if it's not likely in a train car, opt for daytime travel instead.
Train travel gives you the chance to talk to the locals and see things you don't see from the highway or the air. And if you don't feel like talking, slip on the headphones and put your big adventure to music!