Can I use my eurail pass on the AVE train? Is there a fee to reserve a seat on the AVE?
Yes and yes. The high-speed lines all require seat reservations and in my experience, pretty much all mainline trains either require or highly recommend a reservation. For long runs like Madrid-Sevilla or Madrid-Barcelona, the railpass is helpful. For short runs, like Madrid-Toledo, the pass is useless since the reservation is the cost of the ticket.