Re: transportation to trier from frankfurt
You can take a train from the Hauptbahnhof (or from Frankfurt Airport's underground "Regional Bahnhof") to Bingen (sometimes with a change in Mainz). You can then take your boat ride to St. Goar, then a train to Trier, changing trains in Koblenz. Allow 5-6 hours for the excursion, depending on connections.
The St Goar boat dock is about a 15-minute walk from the train station. Trains depart St. Goar for Koblenz (with connections to Trier) hourly.
BTW, If you have a Eurail Pass covering Germany, or a German Rail pass, the river boat ride is also covered.
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