I see the point about it definitely not being a continental corridor, but I don't agree that rendering should be ignored in a mapping effort ;-) moreover, I believe the definition is intentionally vague to allow for considering the context: "higher traffic and higher speed ... longer than a branch line ... electrified" the RFI "dorsale" fulfills that for Sardinia compared to narrow gauge lines. I believe Cagliari-Olbia as "main" and the others as "branches" describes well the network of the island.