Actual no direct train (The Man on seat Sixty one announced a new direct train in few weeks) 

Till then you have to go via Vigo or via another Route (can't get the name yet @Mattheas  Help please ) with several changes 

Yes, the route via Vigo means to stay overnight in Vigo or (if the evening Intercity Vigo - Porto resumes) in Porto.

There is one daytime connection per day, with a change of trains in Cáceres/Puertollano, Badajoz and Entroncamento. The first two trains require a reservation, which can be obtained at the train station in Madrid. (I don't think this connection ever becomes fully booked)

tip: if you need to travel to Madrid Atocha station, you can use your Interrail Pass on the Cercanías (suburban trains) as well. Just ask at the ticket office for a free ticket to pass the turnstiles.

Hi, 

I’ll be travelling between Madrid and Porto in a few weeks. After my initial plan to take the night train fell trough, I decided on two alternative routes. From Spain to Portugal, I’ll be taking the Madrid - Salamanca train and change on to the Flixbus to Porto there. On the return journey I will be departing from Porto to Vigo by train and change there on to the Vigo - Madrid train. It’s much slower but I hope more comfort. 
I’m arriving in Spain trough the port of Santander and leaving again by train via Barcelona and Girona, back home via France. 

Looking forward to it!