Would like to take a fast train from Granada to Madrid in October.
I can't find much info on the internet. Perhaps tickets are not yet available.
Here you go.....https://www.seat61.com/Spain-trains.htm
Generally - seats become available about 60 days prior to your trip.
"Fast Trains" are "AVE" trains (that's the term used in Spain).
This link might help, tambien:
https://www.thetrainline.com/en/train-times/granada-to-madrid?gclid=EAIaIQobChMIlLvhgOXJ4wIVkq_sCh3kWQSlEAAYAiAAEgL-TvD_BwE&gclsrc=aw.ds
Check the correct day of the week for a date as far out as you can find AVEs. The schedule probably will not change much if at all for October.