Trains in Spain

I don't know about the timing of reservation purchases beyond having sometimes seen trains on the schedule marked as full. But do not use the Eurail website to check schedules. It doesn't always list all the options; sometimes you'll only find the most expensive trains there (though that isn't an issue for you since you have the pass).

For a complete listing of trains, you can go to renfe.com. You can take a look now at the options for each of your planned trips on your precise travel dates and see how busy things seem.

"I assume it's offseason".... wrong assumption. High-speed lines weren't built "for tourists" -if you catch my drift- but for local residents having to go from A to B, both for business or for pleasure. Some lines are heavily used on a daily basis (ie Barcelona-Madrid, Madrid-Seville, etc.) so while it doesn't mean all trains are full all the time, you better pre-book or risk not being able to travel on a specific train -especially on certain lines, which include a few of those you listed!

As per when to pre-book, again, that depends on the line and the date/time, ie. early morning trains might be busier -due to commuting- than trains at mid-morning or noon; also trains on certain dates (pre- and post-holiday, and on Monday morning or Friday afternoon) can land you in a bit of a pickle. However, for the most part, and if you have a bit of flexibility -given there are several trains per day in most high speed lines- a few days in advance would suffice for pre-booking. But then, the more you wait, the more expensive they tend to be. Last minute purchases on really busy lines can be a nightmare and bloody expensive.

Spaniards are very mobile so when there's a long holiday, lots of them go to their second residences or just travel elsewhere nationally or internationally. In December you have a very long weekend in the second week as both Wed 6th and Fri 8th are national holidays. A lot of people will take the 7th off so it's likely trains (and planes!) on the evening of the 5th and the morning of the 6th can be busy, as well as on Sun 10th (for the return). Yet those not taking the 7th off but still travelling somewhere will flood the trains in the evening of the 7th and morning of the 8th. In short, a bad weekend for travelling!

Sorry for not being more helpful :)... but train planning is a complex issue here.

Enric, Brad has bought a rail pass, so increasing ticket prices will not be an issue for him, though sold-out trains would be. Does the cost of reservations also increase as the departure approaches? I've never needed to buy a separate reservation, so I have no clue.

You're totally right Ann, thanks for pointing out, forgot about that! About your second question, I don't know, but I wouldn't think. Those 6€ look to me like a sort of "tasa" (fee) for the Eurail, thus fixed. When you pre-book on "regular" tickets, there's no fee (as far as I remember!), you just get charged the amount of the ticket at the moment of pre-booking.

Brad, I just returned from Spain. December is schedule change month for Renfe so they are slow to load schedules. Most of the bigger routes should be up by now, however.
As to Toledo and Segovia schedules, these seem slower to post far in advance, and I did not find as many scheduled as expected, especially for Toledo.
I booked my Toledo train about two weeks prior to use.
Segovia is a little more complicated to find schedules depending if you are determined to leave from Atocha or elsewhere, and they also are posted closer to travel date. I took a slow morning train from Atocha on a Saturday morning booked one hour prior to departure, but coming back I wanted the high speed train. That leaves from a different terminal and only goes to Chamartin station. From there, you need to get to "wherever" by a separately purchased train or metro ticket (1.70€). The trip home I also bought right before travel. I was warned they might be sold out, but there was another train scheduled shortly after, so I took the risk. In fact, I could have booked the train before and saved the wait in a cold train station.
I suggest you weigh the risks of having to wait if a train is sold out vs the cost of reserving longer trips ahead of time.
Seville to Cordoba can be done by bus for the same length of time.

The 6 euro fee is the booking fee from the Eurail website for making the reservation. It is in addition to the actual reservation fee, which is based on the class of travel, time of day, and length of trip.

If you're not already counting on catching a particular departure on each route, then I'd probably buy several reservations at a train station upon arrival in Madrid. You have to get the pass activated there and can do so at the same time. Some travel agencies can also handle this, though I understand that the one in El Corte Ingles department store no longer does.

It sounds as if your trip begins in Madrid. If rail pass activities at Atocha Station must occur in the glassed-in office selling long-distance tickets (I'm not positive about this), you need to allow a good bit of time. That office is busy enough that it uses a take-a-number system, and I've seen them stop giving out numbers 30 minutes or so before closing time because there were so many people waiting.

I’m with Laura.
Make your reservations as a ‘Passholder’ directly with Renfe once you arrive in Spain.
If you want to maintain some flexibility then book your reservation at the station for the onward journey when you arrive in a city. My understanding is the seat reservation fee is a set price for each trip, so it shouldn’t matter when you book, but best to do it a day or two in advance. I would’t think Renfe charges a booking fee like the 6€ with Eurail.