The network of trains through Spain makes it easy to get to a camino starting point from almost any entry port in Spain. The difficulty is pre-purchasing tickets. Computer searches will inevitably direct you to websites of external booking agencies which offer limited options for travel and charge varying hidden booking fees. Buying directly from Renfe is much the best options but it is difficult to negotiate the system.
This website is the best guide I know to understanding the system and getting the tickets you want for the best price.