For trains in both Spain and France, I found that
www.raileurope.com seems to have better information, and the search engine is more user friendly (at least in English) that either Renfe or SNCF. In fact, if you tell SNCF that you are from North America, it defaults over to Rail Europe's site.
This aside, if you are flying from the U.S. or Canada I suggest you consider buying an
"Open Jaw" ticket. You fly from North America to Paris (CDG) then use this information to get to St. Jean Pied de Port:
http://www.caminodesantiago.me/community/threads/a-compendium-of-useful-links-for-travel-paris-to-sjpdp.23274/?utm_source=feedburner&utm_medium=email&utm_campaign=Feed: PilgrimageToSantiagoDeCompostela (Camino de Santiago de Compostela)
(Copy and paste this whole thing into your web browser address bar)
Following your arrival at Santiago, take the ALSA bus or a Renfe train to Madrid. Fly from Madrid (MAD) to your U.S. starting place. This basic outline should also work for our Canadian friends too.
If you are booking online yourself, look for a "multiple-cities," or "multiple-destinations" option next to "Round Trip," and "One-Way." Then enter the two discrete flights (e.g. LAX to CDG then MAD to LAX). The
"Open Jaw" concept has been around for many decades in the air travel industry and it is an accepted practice. So most airline databases are set up to price it directly online.
In my direct experience over 35 years traveling globally, the difference in ticket prices is far less than the effort and cost to return to the original landing place in Europe. Consider that, if you depart Santiago de Compostela in the morning, the earliest you can actually get a return flight to North America is the next day (morning - noon(ish) anyway. So, a night at an airport hotel, or nearby, is a given - and an expense. Madrid is relatively less expensive than the other options: Paris, London, Frankfurt, etc.). Plus, the ALSA coaches are very nice, inexpensive and direct.
I hope this helps.