If I don't want to spend the time understanding the rail schedule, where is the best place to purchase a ticket?
What has your 'Rail pass in spain' to do with your question?
Surely you realise you must understand the rail schedules to know what timed journey so as to purchase a ticket.
What do you want from 'best'? Is this something to do with 'best' price or something else?
The train station? You question is very unclear.