I am ready to purchase rail tickets for use during my upcoming trip to France and Barcelona. I see that some of the morning trains are already sold out on Trainline but are still available on RailEurope. Do these two sites operate the same - purchase and print at home? I see RS people are big on Trainline so I feel pretty comfortable with that. How about RailEurope? Any info would be appreciated.
Yes, Rail Europe has print-at-home options for most French and Spanish tickets. If not, then they would have either print-in-station or home delivery options. They may possibly have been given different "buckets" of seats to sell. They're quite reliable; the only reason not to choose Rail Europe would be if you found a better price with the other agent.
Both Trainline and RailEurope are resellers of train tickets - travel agencies. Much like Orbitz and others in the US with airline tickets. Personally I would go directly to the rail site and purchase the tickets there.
I also book train tickets on the official French Railway website: SNCF.com
You can also save money by booking your tickets in advance.
Edit to add:
You may find this website helpful: http://www.seat61.com
If traveling within Spain, you may book your tickets on the Renfe website which is Spain's Railway Company.