arriving in Madrid a day before I begin in Sarria /Train resv. needed?

A night in Madrid is never wasted. Sarria cannot match the C de San Pedro.

On most weekdays there are trains from Chamartin at 13:05 and 14:57. I haven't looked at weekends. www.Renfe.com will provide.

You haven't said when you arrive in Madrid but I rarely bother with reservations unless I am going to be attempting to travel during a Spanish holiday weekend.

We just did this in April 2019. Booked the the flight into Madrid and train from airport to Sarria via Charmartin station in advance. Didn't work out as planned. Morning flight to Madrid was cancelled and we were re-booked on an evening flight through London. Missed the train and lost money, had to spend an unplanned night in Madrid and buy new train tickets. I know that you can get to Madrid airport and buy your train, and I think also bus, tickets at the airport. Another issue was that they held my pack in London to be delivered in a later flight so, we picked up my wife's pack and waited until late that night to get my pack. I rarely book trains or buses too far in advance but was concerned since we were traveling during Spanish holidays. It all worked out in the end but it did cost a bit of extra money.

Generally the earlier you reserve train tickets the cheaper the tickets but at full price they are still good value for the distance travelled. I like to book early and get a cheap deal on first class, which is nice for a six hour journey.