Reservations for night trains

If you haven't purchased your pass yet, do some checking to see what point to point tickets will cost you. Unless you plan to do a LOT of rail travel, rail passes aren't cost effective, especially if you get some of the specials available on foreign railway websites. For France to Spain, check out http://www.sncf.fr/indexe.htm. Even with a pass, reservations are necessary for many train journeys these days, notably the TGV in France and, as you have found out, the Ellipsos overnight train to Barcelona....most overnight trains require reservations, and they go quickly. For passholders, you can try booking reservations only through places like RailEurope.com or Euraide.com....but they can be very expensive. At this time, the end of March, it may already be too late to get a place on the overnight Ellipsos to Barcelona for May. You may also want to try flying, since discount airlines can be a bargain. For Paris-Barcelona, check out http://easyjet.com and http://vueling.com/EN/, both of which I've used and had good success with.

Yes, from what I have read on this website and in guidebooks, night trains fill up, and you should book those trips in advance.

Check on Rick Steves' web site about buying train tickets, and he tells you how and where to make train reservaitons.