To Spain from Portugal
 

Id like to do the train trip from Porto through to the Duoro valley then go on to Spain instead of retracing my steps to Porto. Is this feasible without a car? Hints?

I'd start by looking at seat61.com
Rome2rio
A map of the train lines in the peninsula
Bahn.de

Then use the local train companies to buy the tickets

Thanks for your suggestions bilboburglar

Not possible without a car or returning to Porto for either bus, train or airplane.