The issue for late May travellers appears to be that there are no scheduled connections between Ourense and Sarria. Trains Madrid to Sarria usually involve changing at Ourense for a train to A Coruna that stops at Sarria. RENFE website is S.H.I.T. (Silly, Hopeless, Incomprehensible, Testing).
If you search for a train from Madrid - Chamartin to A Coruna you will discover that there are two per day, that you will have to change at Ourense and that you get disembark at Sarria. What you are unlikely to be able to do is purchase a ticket from Madrid Chamartin to Sarria until you get to Madrid Chamartin and even there you may have to explain that you want to disembark at Sarria which will cause absolute astonishment in the ticket office that services a couple of hundred pilgrims on their way to Sarria every day.
Es normal.
The Caminos generous inclination to help your personal development can begin long before your first cafe con leche