Unfortunately I think sleeper trains have limited potential. Trains can be faster, more comfortable and more convenient than the alternatives. But sleeper trains are often slower than other trains, they aren't comfortable enough that sleeping in a hotel room wouldn't be much better and often they are more inconvenient since they have to leave and arrive at certain hours.

The reality is that you don't fly to Asia because it's cheap, at least not the ticket, but because many cities in Europe are expensive or dead, or both.