Okay, I went to www.renfe.es which is the website for RENFE (Spanish national passenger rail service), and tried buying a ticket from Barcelona to Pamplona on July 6.  What I discovered was that the RENFE website was a bit of a mess.  As you search for trains in English, it somehow reverts to the registration/log-in page in Castillian.  You have to register with the site before buying a ticket, which is normal (just like Orbitz, EXpedia, etc, as well as France’s SNCF), but the language of the user’s preference is not constant throughout the process.

I searched the web for some info, and apparently non-nationals and non-residents of Spain should use the English log-in/registration even if you speak Castillian.  Apparently, the Castillian site asks for DNI (Spanish national ID number; NIF is an individual’s tax number. http://www.spainexpat.com/spain/information/id_numbers_in_spain/). 

So, as you search for trains, then click on “Tiknet” to purchase the ticket you want, it takes you to a log-in/registration page in Castillian.  When you get to that point, click on the box on the top right that says “English” to get to an information page in English, and then from the menu of the left under “Sales” click on “Ticket Sales” to get to the registration/log-in page in English.  Here, it simply asks for “ID number” which is probably just used for identification purposes.  I found some advice on the internet suggesting to enter your passport number, minus the last digit. http://www.tripadvisor.com/ShowTopic-g187427-i42-k836365-Help_what_is_N_I_F_needed_to_book_train_tix_w_RENFE-Spain.html 

Try that, and if it doesn’t work, try taking off the two last digits (because the RENFE website asked me for DNI, which is one digit shorter than NIF).

So, try this, and let us know how it works out.
 
If worst comes to worst, you’ll just have to go the travelocity way which is worth the $75 in my opinion, if this is something you really want to do.