You might find this below helpful -- an answer to your question I just chanced upon it earlier today since we are also looking at transport from Madrid to Sarria. This question was posted back in August, so can't vounch for its timeliness or reliability>
posts: 2 Transportation from Madrid to Sarria
Aug 14, 2012, 3:13 PM
Does anyone know how to get from Madrid to Sarria, which is our entry point for walking the Camino to Santiago? I can't seem to find it on the train (RENFE) schedule.
1. Re: Transportation from Madrid to Sarria
Aug 14, 2012, 3:36 PM
Hi Carol Q,
I think that the best option is a train from Madrid to Monforte de Lemos and bus from Monforte to Sarria.
From Madrid you can take a couple of trains to Monforte de Lemos, both departing from Chamartín Station (11:40 and 22:30 - journey around 7 hours 45 minutes). I am not sure if both run daily. :-/
From Monforte de Lemos you have to take a bus to Sarria (buses run from 6:35 to 22:05 - journey between 30 and 45 minutes).