There is no train to Positano. No metro, no rail. You're getting there by bus (winding cliff road), ferry (seasonal), or car (bad idea). The Amalfi Coast was built for goats and fishermen, not 5 million annual tourists.
Alilauro and NLG from Naples Molo Beverello. The approach to Positano from the sea is unforgettable. Book on alilauro.it — summer ferries sell out by mid-morning.
Circumvesuviana to Sorrento (1h10, €4.20) then SITA bus (50 min, €2.50). Standing room only in summer, hairpin turns, stunning views. Sit on the right for sea views.
Door-to-door for up to 4 people. Book on GetYourGuide or Viator. Worth it if splitting among a group.
Ferry if running (April–October). SITA bus via Sorrento if not. Don't drive — the road is misery.