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It seems like internorte.pt does not show results from Santiago to Porto anymore.
If booking directly with ALSA still brings problems, you can try booking with Busbud.
In addition to ALSA, Flixbus is currently offering this route.
Other options would be car-sharing platform BlaBlaCar or taking...
For further reference, in case somebody has the same question.
Regarding buses, ALSA, Flixbus and Eurolines cover this route.
For trains, one would first need to go to Vigo with Renfe (Spanish Railways) and then change to another train with Comboios de Portugal (Portuguese Railways). Loco2.com...
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