Hi
Not quite the answer you were looking for but I did the London-Paris Eurostar then the Paris-Madrid Elipsos sleeper this Summer, just me and my 8 yo daughter.
It was great. Nice ambiance with fellow passengers and the staff were very helpful and efficient. Restaurant car was lovely (I wish I had booked it) and the cafe was nice although busy at peak periods. Im a real scaredy cat but my experience was so positive that yes, I would consider the 4-bunk option if I were on my own.
There is absolutely no way I would do Eurolines - nothing against the service but I love trains. Someone wise once said - there is travel, and there is air transport, but there is no such thing as air travel!! I would extend this to coaches - I had fabulous views that I wouldnt get in 12hrs of motorway....
If you havent seen it, try www.seat61.com for non-flyers. Its my bible now and I am looking at Chicago this year. No planes involved!!
Good luck - have a fab time.