There is no train to Holwerd. The nearest rail connection is Leeuwarden, from where buses run to Holwerd in roughly 45 minutes. Bus line 66 (operated by Arriva) connects Leeuwarden station with the Holwerd ferry terminal — check current timetables as services are designed to connect with ferry departures.
By car, take the N356 north from Leeuwarden. The drive takes about 30 minutes. Parking is available near the ferry terminal.
If you are coming from Groningen or the east, Dokkum is the nearest town on the way, about 15 minutes south of Holwerd.