Upvote:4
Have a look at the website The Man in Seat 61.
He's tried to put together an incredible wealth of information on train travel, the options, routes, and facilities of different journeys around the world.
I used it for European travel, a trip last year across America, and for planning of my Russia travels as well. Very handy reference resource!
Upvote:4
The best online listing of sleeper trains I know of is RailDude. For example the Germany page http://www.raildude.com/en/germany/country/de lists all the night trains available passing through Germany, you can click on one to see the timetable, type of accomodation, prices etc.
seat61 has a lot of info but it's more of a guidebook than a listing of trains, and is focused on getting from the UK to places rather then getting around Europe.
Another option is to purchase a copy of the European Rail Timetable(search google for 'European Rail Timetable')
Finally, for checking specific routes go to http://reiseauskunft.bahn.de/bin/query2.exe/en and just search the route(the site is run by Deutsche Bahn but covers most of Europe), giving a departure time in the evening. Sleeper trains will come up as EN(Euronight), CNL(CityNightLine) or D in the Product Column