The yellow line, line D, goes over the Dom Luis I bridge (built by Georg Eiffel) to Vila Nova da Gaia The stop just before the bridge(stop São Bento) is the closest to the Sé cathedral. Coming from the airport you can stay on the same train to Trindade . For São Bento you have to change at Trindade station too .
I prefer to travel to the station Trindade and walk from there to the cathedral .
You pass the praça da Liberdade with the only place in the world where water has no "spirit level"
And look at the typical Portuguese tiles in the hall of the world famous São Bento railway station. From Trindade it is an easy and relaxed walk to the Sé