By train: Koblenz Hauptbahnhof, ten minutes' walk south of the Altstadt, is on the West Rhine main line — direct ICE trains to Cologne (50 minutes), Frankfurt (90 minutes), and Mainz (45 minutes). The slower Mittelrhein-Bahn runs the same route along the gorge and is a beautiful trip in either direction.
By air: Frankfurt-Hahn ("Frankfurt" only in the loosest sense) is the closest airport at one hour by bus. Frankfurt Main is 90 minutes by train; Cologne-Bonn is 90 minutes by train.
By boat: The KD line runs daily summer services up the gorge to Rüdesheim and down to Bonn, both around four hours each way. A combined train-and-boat day ticket is the cheapest way to see the Rhine Gorge.