How do you book a sleeper car with rail europe?

On the Rail Europe website how do you select a sleeping car, or make the choice between sleeper, couchette, regular seat etc.
Or is there a better way to book (mainly eastern europe)

On the raileurope website, you can only buy the full ticket, and if you’re booking a night train, click on details and it tells you whether you’re in a T6 or T4 couchette compartment, and if you click on the most expensive it’s probably a better compartment. The only problem is if you have a railpass, there’s no way just to reserve and pay the supplement for a couchette. My advice is to go to a train station as soon as you get to Europe and book the reservations there. If you have a railpass, show it to them and they will only charge you the supplement. If a train station can’t do it for a train in another country, try a travel agency in Europe that sells train tickets. I was able to pay for a couchette supplement in Amsterdam for a train in Germany. In some cases you can just book it the morning of the departure, but popular routes can sell out, so it’s better to go a day or two in advance. There’s a great wwebsite about train travel that might have some info for you. http://www.seat61.com